Fix IDAKLU output_variables to work with extrapolation events

[pipliggins]

Fixes #4439

Similarly to #3937 this is caused by code searching for variables in the full state vector which aren't present due to the reduced set produced when output_variables are specified.

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.46%. Comparing base (7be637c) to head (f5717ff).
Report is 9 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #4440   +/-   ##
========================================
  Coverage    99.46%   99.46%           
========================================
  Files          293      293           
  Lines        22381    22384    +3     
========================================
+ Hits         22261    22264    +3     
  Misses         120      120           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[MarkVeerasingam]

@pipliggins - just tried your edit, everything works well on my end, thanks for your help on this! :)