Add functions of time in experiment step

[MarcBerliner]

Description

Adds functionality to pass in arbitrary functions of time as the argument for a pybamm.step

Fixes #4187

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) (I haven't gotten idaklu installed so I'm not certain all tests pass)
• 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

[valentinsulzer]

Thanks, this is perfect, just need tests for coverage

[valentinsulzer]

This should be made into a new section as it won't be included in this release

[MarcBerliner]

This is under Unreleased right now. Do you want me to move everything else under Unreleased into a new section?

[valentinsulzer]

Yes - though let's check with release team @Saransh-cpp @agriyakhetarpal @kratman to see the best way to deal with this

[agriyakhetarpal]

Since this PR is going into the develop branch and is a feature addition, the merged commit here won't be cherry-picked into further release candidates – it will go straight into v24.9. I think it should be fine to leave as is. Ideally, cherry-picked bug fixes will carry their own CHANGELOG entries into the next release candidate or final release.

Since the release notes are still copied over manually, we'll have a chance there to check for unneeded entries

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.55%. Comparing base (e22d10c) to head (e6decee).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #4222   +/-   ##
========================================
  Coverage    99.55%   99.55%           
========================================
  Files          288      288           
  Lines        21856    21871   +15     
========================================
+ Hits         21759    21774   +15     
  Misses          97       97           

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[MarcBerliner]

For steps that compute their charge/discharge direction (Current, CRate, Power, Resistance), there were a few redundancies in the BaseStep initialization. I moved the value_based_charge_or_discharge inside the BaseStep class and evaluate it after sanitizing the step value. This should fix the codecov (I believe)

[rtimms]

Looks great, thanks!