refactor experiment steps to allow customization by user

[valentinsulzer]

Description

Refactor experiment steps to move the logic for processing the step from the Simulation class to the Step class.
This allows users to create custom steps by writing their own logic, using the CustomStepExplicit or CustomStepImplicit classes.

Fixes #3530

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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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.60%. Comparing base (280da52) to head (3670294).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3835   +/-   ##
========================================
  Coverage    99.60%   99.60%           
========================================
  Files          259      259           
  Lines        21275    21331   +56     
========================================
+ Hits         21191    21247   +56     
  Misses          84       84           

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[rtimms]

looks great, thanks!