API documentation for fenicsx branch

[jhdark]

Proposed changes

With this change, all the classes and functions in the fenicsx branch are documented, and any example usage in the doc strings is tested.

The API has been implemented in such a way so that any additional classes created within the same files should be automatically added. Unless, a new file is created within one of the existing folders, in which case, the class needs to be added to the '_all' list in the 'init.py'

Types of changes

What types of changes does your code introduce to FESTIM?

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Code refactoring
• Documentation Update (if none of the other choices apply)
• New tests

Checklist

• Black formatted
• Unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.91%. Comparing base (12a4682) to head (270028b).
Report is 121 commits behind head on fenicsx.

Additional details and impacted files

@@             Coverage Diff             @@
##           fenicsx     #976      +/-   ##
===========================================
+ Coverage    95.89%   95.91%   +0.02%     
===========================================
  Files           46       46              
  Lines         2705     2719      +14     
===========================================
+ Hits          2594     2608      +14     
  Misses         111      111              

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[RemDelaporteMathurin]

For some reason the links to the API don't seem to work https://festim--976.org.readthedocs.build/en/976/userguide/subdomains.html