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 https://github.com/pybamm-team/PyBaMM/issues/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.

• [x] 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:

• [x] 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)
• [x] 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:

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

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

:umbrella: View full report in Codecov by Sentry.
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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)