Issue 3530 custom termination

[valentinsulzer]

Description

Adds ability to create custom termination events, using any variable, for an experiment step

First part of #3530 (second part is using custom steps, not just terminations)

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

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[rtimms]

Thanks, looks good!

Not for this issue, but the mixed step/string definitions of experiments is a little weird. I think users like the string experiments and dropping them would be a mistake.

[valentinsulzer]

Not for this issue, but the mixed step/string definitions of experiments is a little weird. I think users like the string experiments and dropping them would be a mistake.

Yeah there are good reasons to keep both, what are you suggesting?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (32fad00) 99.58% compared to head (3468e08) 99.59%.
Report is 36 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3596   +/-   ##
========================================
  Coverage    99.58%   99.59%           
========================================
  Files          257      258    +1     
  Lines        20708    20755   +47     
========================================
+ Hits         20623    20670   +47     
  Misses          85       85           

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[rtimms]

Yeah there are good reasons to keep both, what are you suggesting?

Nothing for now, but I'm just conscious it may be confusing for new users to see both ways of doing things within the same experiment definition.

[brosaplanella]

Looks good, happy to merge once coverage and tests are fixed! About the interface, I think it should be OK to maintain the string option for a subset of functionality.