Expression: split out evaluate_params

[mathause]

• towards frozen distribution considerably slower #532
• Tests added
• Fully documented, including CHANGELOG.rst

This creates a separate method to evaluate_params - so we don't necessarily create the frozen distribution. E.g. if we only need params["loc"] we can save the slow creation of the distribution.

This saves about 1-2 minutes on the tests already (about 20% - 25%).

Only a draft PR for now

• it's on top of Expression.evaluate: avoid exec by using locals #535 - so waiting for that to be merged first
• the tests need to be updated
• needs docs

[codecov]

Codecov Report

Attention: Patch coverage is 95.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 61.93%. Comparing base (685a5fd) to head (a7de4cb).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
mesmer/mesmer_x/train_utils_mesmerx.py	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #539      +/-   ##
==========================================
+ Coverage   61.89%   61.93%   +0.03%     
==========================================
  Files          50       50              
  Lines        3551     3554       +3     
==========================================
+ Hits         2198     2201       +3     
  Misses       1353     1353              

Flag	Coverage Δ
unittests	61.93% <95.00%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[mathause]

Thanks

[mathause]

@l-pierini with this change merged you should see a performance improvement for the fitting (more to come but I don't know when)