Issue 2040 events

[valentinsulzer]

Description

Raises a SolverError if the initial conditions trigger the events. This requires the events to always be written in a way that they are positive initially, and should be triggered when they become negative.

In an experiment, if an "[experiment]" event is triggered, then the step is just skipped (returns EmptySolution) without raising an error. This is to deal with cases such as "Charge at 1C until 4.2V" when we start from full SOC.

Fixes #2040

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #) - note reverse order of PR #s. If necessary, also add to the list of breaking changes.

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist:

• No style issues: $ flake8
• All tests pass: $ python run-tests.py --unit
• The documentation builds: $ cd docs and then $ make clean; make html

You can run all three at once, using $ python run-tests.py --quick.

Further checks:

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

Merging #2212 (824f968) into develop (f550813) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #2212   +/-   ##
========================================
  Coverage    99.39%   99.40%           
========================================
  Files          364      364           
  Lines        19930    20007   +77     
========================================
+ Hits         19810    19887   +77     
  Misses         120      120           

Impacted Files	Coverage Δ
...m/models/full_battery_models/base_battery_model.py	99.78% <ø> (ø)
...models/full_battery_models/lead_acid/basic_full.py	100.00% <ø> (ø)
...odels/full_battery_models/lithium_ion/basic_dfn.py	100.00% <ø> (ø)
..._battery_models/lithium_ion/basic_dfn_half_cell.py	99.01% <ø> (ø)
...odels/full_battery_models/lithium_ion/basic_spm.py	100.00% <ø> (ø)
.../submodels/particle_mechanics/crack_propagation.py	100.00% <ø> (ø)
pybamm/models/submodels/porosity/base_porosity.py	100.00% <ø> (ø)
pybamm/__init__.py	100.00% <100.00%> (ø)
pybamm/expression_tree/exceptions.py	100.00% <100.00%> (ø)
pybamm/simulation.py	98.86% <100.00%> (+0.02%)	⬆️
... and 3 more

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[brosaplanella]

Looks good, thanks!