Improve external dependency handling of `PipelineStepMixin`

[synchon]

• [x] description of feature/fix
• [x] tests added/passed
• [x] add an entry for the latest changes

This PR improves the external dependency handling of PipelineStepMixin derived objects like Markers and Preprocessors. It adds a new class variable called _CONDITIONAL_DEPENDENCIES which interact with _DEPENDENCIES and _EXT_DEPENDENCIES. This enables easy validation and usage of objects which have multiple implementations like ReHo, ALFF and BOLDWarper. It also sets a nice foundation for similar objects in the future as will be implemented in #161 and #301. It also refactors internal code wherever required.

[github-actions[bot]]

PR Preview Action v1.4.7 :---: Preview removed because the pull request was closed. 2024-03-14 14:31 UTC

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 78.23529% with 74 lines in your changes are missing coverage. Please review.

Project coverage is 88.55%. Comparing base (5540b36) to head (8bf68ac).

Additional details and impacted files

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[synchon]

Can you also update docs/extending/markers.rst? Maybe create a small section on "dependencies" and then refer to that?

This way, we will document everything that is happening on this PR (which was also a bit undocumented before)

I'm already updating the docs for the release at https://github.com/juaml/junifer/tree/update/improve-docs . Will address your suggestions there along with a few more things that I noticed aren't there.