Add ordinal families

[tomicapretto]

This PR adds two new families: "cumulative" and "sratio". These are directly inspired by their versions in brms. For more information about them see the excellent tutorial Ordinal Regression Models in Psychology: A Tutorial.

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[codecov-commenter]

Codecov Report

Merging #678 (10b409c) into main (b83f545) will increase coverage by 0.64%.
The diff coverage is 97.23%.

@@            Coverage Diff             @@
##             main     #678      +/-   ##
==========================================
+ Coverage   88.01%   88.65%   +0.64%     
==========================================
  Files          40       40              
  Lines        2678     2847     +169     
==========================================
+ Hits         2357     2524     +167     
- Misses        321      323       +2     

Impacted Files	Coverage Δ
bambi/defaults/families.py	87.50% <ø> (ø)
bambi/families/likelihood.py	85.36% <ø> (ø)
bambi/priors/prior.py	86.11% <44.44%> (-13.89%)	⬇️
bambi/formula.py	94.73% <92.30%> (-1.82%)	⬇️
bambi/backend/model_components.py	100.00% <100.00%> (ø)
bambi/backend/terms.py	96.47% <100.00%> (ø)
bambi/backend/utils.py	90.62% <100.00%> (+0.96%)	⬆️
bambi/families/family.py	94.79% <100.00%> (+0.22%)	⬆️
bambi/families/multivariate.py	97.05% <100.00%> (+0.04%)	⬆️
bambi/families/univariate.py	97.61% <100.00%> (+1.81%)	⬆️
... and 4 more

... and 2 files with indirect coverage changes

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