3937 events with idaklu output variables - solver edit

[pipliggins]

Description

This is an alternative to #4150 (old pull request will be closed)

Simulations using the IDAKLU solver with output variables specified which terminated with an event rather than the final time step would crash trying to find the termination event.

Termination events are found in the Solution class by using the state vector of the final time-step to evaluate each potential termination event and find the minimum (the event which caused the termination). When output variables are specified in the IDAKLU solver only those variables are output in y_out, not the entire state vector, so the events could not be evaluated outside the solver.

This PR edits the IDAKLU solver to add an additional return value containing the state vector for the final timestep only ('y_term') when output variables are specified. This value is used to fill the Solution.y_event attribute and allows termination events to be calculated as normal, while not impacting the performance advantages of selecting only a few output variables. When output variables are not specified, this returns an empty array as it copies part of the provided state vector.

Fixes #3937

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: $ pre-commit run (or $ nox -s pre-commit) (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• All tests pass: $ python run-tests.py --all (or $ nox -s tests)
• The documentation builds: $ python run-tests.py --doctest (or $ nox -s doctests)

You can run integration tests, unit tests, and doctests together at once, using $ python run-tests.py --quick (or $ nox -s quick).

Further checks:

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

[pipliggins]

As mentioned in the previous PR (#4150 (comment)), specifying output variables in the IDAKLU solver doesn't currently work with experiments. This bug fix should enable that behaviour to be added, but I think it should be a separate issue/PR for a new feature, rather than added to this one.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.45%. Comparing base (5408ce3) to head (167dd15).
Report is 161 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #4300      +/-   ##
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- Coverage    99.45%   99.45%   -0.01%     
===========================================
  Files          288      288              
  Lines        22092    22091       -1     
===========================================
- Hits         21972    21971       -1     
  Misses         120      120              

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[jsbrittain]

Looks good to me.

[martinjrobins]

thanks @pipliggins, looks good

[martinjrobins]

@all-contributors please add @pipliggins for code and test

[allcontributors]

@martinjrobins

I've put up a pull request to add @pipliggins! 🎉