Add to_pymc method to distributions

[aloctavodia]

Closes #517

This allows us to convert PreliZ distribution to PyMC distributions. This will work both inside and outside a model context.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 83.33333% with 3 lines in your changes missing coverage. Please review.

Project coverage is 82.16%. Comparing base (8f36b1e) to head (0a35ba2).
Report is 29 commits behind head on main.

Files	Patch %	Lines
preliz/distributions/distributions.py	72.72%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #523      +/-   ##
==========================================
- Coverage   82.36%   82.16%   -0.20%     
==========================================
  Files          96      100       +4     
  Lines        7895     7975      +80     
==========================================
+ Hits         6503     6553      +50     
- Misses       1392     1422      +30     

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