Add helper functions to calculate required M for hsgp

[ferrine]

It is always a trouble to calculate required M for HSGP, adding helper functions to simplify that

What is this PR about? ...

Checklist

• [x] Explain important implementation details 👆
• [x] Make sure that the pre-commit linting/style checks pass.
• [ ] Link relevant issues (preferably in nice commit messages)
• [x] Are the changes covered by tests and docstrings?
• [x] Fill out the short summary sections 👇

Major / Breaking Changes

• ...

New features

• Calculating HSGP m parameter for common kernels

Bugfixes

• ...

Documentation

• ...

Maintenance

• ...

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:books: Documentation preview :books:: https://pymc--7058.org.readthedocs.build/en/7058/

[codecov[bot]]

Codecov Report

Merging #7058 (9f18c43) into main (005ba5f) will decrease coverage by 0.70%. Report is 2 commits behind head on main. The diff coverage is 92.30%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7058      +/-   ##
==========================================
- Coverage   92.16%   91.47%   -0.70%     
==========================================
  Files         101      101              
  Lines       16827    16840      +13     
==========================================
- Hits        15509    15404     -105     
- Misses       1318     1436     +118     

Files	Coverage Δ
pymc/gp/cov.py	97.84% <92.30%> (-0.16%)	:arrow_down:

... and 3 files with indirect coverage changes

[ricardoV94]

Marked as a draft given all requests for changes in the review. Would be great to get this one over the finish line!