Second draft for contribution guide

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+# VSCode
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CHANGELOG.md

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+# Changelog
+
+:arrow_left: [Back to main page](./README.md)
+
+-   October 7, 2022: Version 5 comes with many changes, both visible and
+    invisible ones. Some of those break the existing API, but if you are
+    using tests, you should be fine. Major changes include:
+
+    -   **New convergence controllers**: Checking whether a step has
+        converged can be tricky, so we made separate modules out of
+        these checks. This makes features like adaptivity easier to
+        implement. Also, the controllers have been streamlined a bit to
+        make them more readable/digestible. Thanks to
+        [\@brownbaerchen](https://github.com/brownbaerchen)!
+    -   **Adaptivity and error estimators**: SDC now comes with
+        adaptivity and error estimation, leveraging the new convergence
+        controllers out of the box. Thanks to
+        [\@brownbaerchen](https://github.com/brownbaerchen)!
+    -   **New collocation classes**: We completely rewrote the way
+        collocation nodes and weights are computed. It is now faster,
+        more reliable, shorter, better. But: this **breaks the API**,
+        since the old collocation classes do not exist anymore. The
+        projects, tutorials, tests and most of the playgrounds have been
+        adapted, so have a look over there to see [what to
+        change](https://github.com/Parallel-in-Time/pySDC/commit/01ffabf71a8d71d33b74809271e8ad5a7b03ac5e#diff-adf74297b6c64d320f4da0f1d5528eda6229803a6615baf5d54c418032543681).
+        Thanks to [\@tlunet](https://github.com/tlunet)!
+    -   **New projects**: Resilience and energy grid simulations are
+        ready to play with and are waiting for more ideas! We used this
+        effort to condense and clean up the problem classes a bit,
+        reducing the number of files and classes with only marginal
+        differences significantly. This could potentially **break your
+        code**, too, if you rely on any of those affected ones. Thanks
+        to [\@brownbaerchen](https://github.com/brownbaerchen) and
+        [\@lisawim](https://github.com/lisawim)!
+    -   **Toward GPU computing**: We included a new data type based on
+        [CuPy](https://cupy.dev/) making GPU computing possible. Thanks
+        to [\@timo2705](https://github.com/timo2705)!
+    -   **Better testing**: The CI pipeline got a complete overhaul
+        (again), now enabling simultaneous tests, faster/earlier
+        linting, benchmarking (at least, in principal), separate
+        environments and so on. The code is tested under Ubuntu and
+        MacOS.
+    -   **Better code formatting**: `pySDC` now uses
+        [black](https://black.readthedocs.io) and
+        [flakeheaven](https://flakeheaven.readthedocs.io) for cleaner
+        source code. After complaints here and there about linting
+        \"errors\" the recommended way now is to run `black` before
+        submission.
+
+-   December 13, 2021: Version 4.2 brings compatibility with Python 3.9,
+    including some code cleanup. The CI test suite seems to run faster
+    now, since sorting out the dependencies is faster. Tested
+    [mamba](https://github.com/mamba-org/mamba), which for now makes the
+    CI pipeline much faster. Also, the CI workflow can now run locally
+    using [act](https://github.com/nektos/act). We introduced markers
+    for soem of the tests in preparation of distributed tests on
+    different platforms. And finally, a LaTeX installation is no longer
+    needed use plotting (but recommended).
+
+-   August 11, 2021: Version 4.1 has some more changes under the hood,
+    most of them with no significant impact to users. The CI pipeline
+    has been completely rewritten, porting the code to [Github
+    Actions](https://github.com/features/actions) (away from [Travis
+    CI](https://travis-ci.com/)), to [flake8](https://flake8.pycqa.org)
+    and to [pytest](https://pytest.org) (away from
+    [nose](https://nose.readthedocs.io)). One thing that may have an
+    impact on users is that following the changes made in Version 4.0,
+    the PETSc data structures are now much easier, removing a lot of
+    unnecessary boilerplate code.
+
+-   May 4, 2021: Long time, no see, but this major release 4.0 marks
+    some improvements under the hood:
+
+    -   **Rewrote `mesh` and `particle` data type**:
+        Creation of new arrays for each operation is now avoided by
+        directly subclassing Numpy\'s `ndarray`. Somewhat faster,
+        definitively better, less code, future-proof, but also breaking
+        the API. If you use `pySDC` for your project, make
+        sure you adapt to the new data types (or don\'t upgrade).
+    -   **Faster quadrature**: Thanks to
+        [tlunet](https://github.com/tlunet) the computation of the
+        weights is now faster and (even) more reliable. No breaking of
+        any API here\...
+    -   **Bugfixing and version pushes**: The code should run without
+        (many) complaints with Python 3.6, 3.7 and potentially above.
+        Also, the plotting routines have been adapted to work with
+        recent versions of `matplotlib`.
+
+    This is not much (yet) and if it were not for the API changes, this
+    would have been a minor release.
+
+-   August 30, 2019: Version 3.1 adds many more examples like the
+    nonlinear Schrödinger equation, more on Gray-Scott and in particular
+    Allen-Cahn. Those are many implemented using the parallel FFT
+    library
+    [mpi4pi-fft](https://bitbucket.org/mpi4py/mpi4py-fft/src/master/),
+    which can now be used with `pySDC`. There are now 8
+    tutorials, where step 7 shows the usage of three external libraries
+    with `pySDC`: mpi4py, FEniCS and petsc4py. The MPI controller has
+    been improved after performaning a detailed performance analysis
+    using [Score-P](https://www.vi-hps.org/projects/score-p/) and
+    [Extrae](https://www.vi-hps.org/Tools/Extrae.html). Finally: first
+    steps towards error/iteration estimators are taken, too.
+
+-   February 14, 2019: Released version 3 of `pySDC`. This
+    release is accompanied by the **ACM TOMS paper**
+    ["pySDC -- Prototyping spectral deferred corrections"](https://doi.org/10.1145/3310410).
+    It release contains some breaking changes to the API. In detail:
+
+    -   **Dropped Python 2 support**: Starting with this version,
+        `pySDC` relies on Python 3. Various incompabilities
+        led to inconsistent treatment of dependencies, so that parts of
+        the code had to use Python 2 while other relied on Python 3 or
+        could do both. We follow [A pledge to migrate to Python
+        3](https://python3statement.org/) with this decision, as most
+        prominent dependencies of `pySDC` already do.
+    -   **Unified controllers**: Instead of providing (and maintaining)
+        four different controllers, this release only has one for
+        emulated and one for MPI-based time-parallelization
+        (`controller_nonMPI` and `controller_MPI`). This should avoid
+        further confusion and makes the code easier to maintain. Both
+        controllers use the multigrid perspective for the algorithm
+        (first exchange data, than compute updates), but the classical
+        way of determining when to stop locally (each time-step is
+        stopped when ready, if the previous one is ready, too). The
+        complete multigrid behavior can be restored using a flag. All
+        included projects and tutorials have been adapted to this.
+    -   **No more data types in the front-ends**: The redundant use of
+        data type specifications in the description dictionaries has
+        been removed. Data types are now declared within each problem
+        class (more precisely, in the header of the `__init__`-method to
+        allow inhertiance). All included projects and tutorials have
+        been adapted to this.
+    -   **Renewed FEniCS support**: This release revives the deprecated
+        [FEniCS](https://fenicsproject.org/) support, now requiring at
+        least FEniCS 2018.1. The integration is tested using Travis-CI.
+    -   **More consistent handling of local initial conditions**: The
+        treatment of `u[0]` and `f[0]` has been fixed and made
+        consistent throughout the code.
+    -   As usual, many bugs have been discovered and fixed.
+
+-   May 23, 3018: Version 2.4 adds support for
+    [petsc4py](https://bitbucket.org/petsc/petsc4py)! You can now use
+    [PETSc](http://www.mcs.anl.gov/petsc/) data types
+    (`pySDC` ships with DMDA for distributed structured
+    grids) and parallel solvers right from your examples and problem
+    classes. There is also a new tutorial (7.C) showing this in a bit
+    more detail, including communicator splitting for parallelization in
+    space and time. Warning: in order to get this to work you need to
+    install petsc4py and mpi4py first! Make sure both use MPICH3
+    bindings. Downloading `pySDC` from PyPI does not include
+    these packages.
+
+-   February 8, 2018: Ever got annoyed at `pySDC`\'s
+    incredibly slow setup phase when multiple time-steps are used?
+    Version 2.3 changes this by copying the data structure of the first
+    step to all other steps using the [dill
+    Package](https://pypi.python.org/pypi/dill). Setup times could be
+    reduced by 90% and more for certain problems. We also increase the
+    speed for certain calculations, in particular for the Penning trap
+    example.
+
+-   November 7, 2017: Version 2.2 contains matrix-based versions of
+    PFASST within the project `matrixPFASST`. This involved quite a few
+    changes in more or less unexpected places, e.g. in the multigrid
+    controller and the transfer base class. The impact of these changes
+    on other projects should be negligible, though.
+
+-   October 25, 2017: For the [6th Workshop on Parallel-in-Time
+    Integration](https://www.ics.usi.ch/index.php/6th-workshop-on-parallel-in-time-methods)
+    `pySDC` has been updated to version 2.1. It is now
+    available on PyPI - the Python Package Index, see
+    <https://pypi.python.org/pypi/pySDC> and can be installed simply by
+    using `pip install pySDC`. Naturally, this release contains a lot of
+    bugfixes and minor improvements. Most notably, the file structure
+    has been changed again to meet the standards for Python packaging
+    (at least a bit).
+
+-   November 24, 2016: Released version 2 of `pySDC`. This
+    release contains major changes to the code and its structure:
+
+    -   **Complete redesign of code structure**: The `core` part of
+        `pySDC` only contains the core modules and classes,
+        while `implementations` contains the actual implementations
+        necessary to run something. This now includes separate files for
+        all collocation classes, as well as a collection of problems,
+        transfer classes and so on. Most examples have been ported to
+        either `tutorials`, `playgrounds` or `projects`.
+    -   **Introduction of tutorials**: We added a tutorial (see below)
+        to explain many of pySDC\'s features in a step-by-step fashion.
+        We start with a simple spatial discretization and collocation
+        formulations and move step by step to SDC, MLSDC and PFASST. All
+        tutorials are accompanied by tests.
+    -   **New all-inclusive controllers**: Instead of having two PFASST
+        controllers which could also do SDC and MLSDC (and more), we now
+        have four generic controllers which can do all these methods,
+        depending on the input. They are split into two by two class:
+        `MPI` and `NonMPI` for real or virtual
+        parallelisim as well as `classic` and
+        `multigrid` for the standard and multigrid-like
+        implementation of PFASST and the likes. Initialization has been
+        simplified a lot, too.
+    -   **Collocation-based coarsening** As the standard PFASST
+        libraries [libpfasst](https://bitbucket.org/memmett/libpfasst)
+        and [PFASST++](https://github.com/Parallel-in-Time/PFASST)
+        `pySDC` now offers collocation-based coarsening,
+        i.e. the number of collocation nodes can be reduced during
+        coarsening. Also, time-step coarsening is in preparation, but
+        not implemented yet.
+    -   **Testing and documentation** The core, implementations and
+        plugin packages and their subpackages are fully documented using
+        sphinx-apidoc, see below. This documentation as well as this
+        website are generated automatically using
+        [Travis-CI](https://travis-ci.org/Parallel-in-Time/pySDC). Most
+        of the code is supported by tests, mainly realized by using the
+        tutorial as the test routines with clearly defined results.
+        Also, projects are accompanied by tests.
+    -   Further, minor changes:
+        -   Switched to more stable barycentric interpolation for the
+            quadrature weights
+        -   New collocation class: `EquidistantSpline_Right`
+            for spline-based quadrature
+        -   Collocation tests are realized by generators and not by
+            classes
+        -   Multi-step SDC (aka single-level PFASST) now works as
+            expected
+        -   Reworked many of the internal structures for consistency and
+            simplicity
+
+:arrow_left: [Back to main page](./README.md)