Add constants method

[nstarman]

A convenience method to access the constants used by the cosmology.
I've been using the API and this has been a pain point.

PR Checklist

• Give a detailed description of the PR above.
• Document changes in the CHANGES.rst file. See existing changelog for examples.
• Add tests, if applicable, to ensure code coverage never decreases.
• Make sure the docs are up to date, if applicable, particularly the docstrings and RST files in docs folder.
• Ensure linear history by rebasing, when requested by the maintainer.

[codecov-commenter]

Codecov Report

Merging #45 (61f160a) into main (917f2cb) will decrease coverage by 0.04%.
The diff coverage is 60.00%.

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@@            Coverage Diff             @@
##             main      #45      +/-   ##
==========================================
- Coverage   77.19%   77.16%   -0.04%     
==========================================
  Files          13       13              
  Lines         285      289       +4     
==========================================
+ Hits          220      223       +3     
- Misses         65       66       +1     

Impacted Files	Coverage Δ
src/cosmology/api/_core.py	80.00% <60.00%> (-1.25%)	⬇️

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