Add negative-binomial conjugate rewrite

[xjing76]

This PR will add in NB-beta conjugates along with its tests.

Because the at.random.negative_binomial takes in the number of successes and I switched the the formula for beta and alpha suggested here

This PR is working towards Issue#48

[xjing76]

I think .github/workflow was not up to date before which was causing the failing of test/py3.10. should be fixed now.

[brandonwillard]

I think .github/workflow was not up to date before which was causing the failing of test/py3.10. should be fixed now.

Yeah, I think we need #94 to fix that. I'll set up some xfails to get past the current issue in that PR, then we can push its fixes through and keep things moving.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.50%. Comparing base (50b7068) to head (087df87).
Report is 22 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #105      +/-   ##
==========================================
+ Coverage   98.16%   98.50%   +0.34%     
==========================================
  Files          10       10              
  Lines         708      737      +29     
  Branches       63       63              
==========================================
+ Hits          695      726      +31     
+ Misses          5        4       -1     
+ Partials        8        7       -1     

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[rlouf]

I fixed the formatting issues and added a test to make sure the rewrite is picked by construct_sampler.

[brandonwillard]

Looks good, just some minor formatting recommendations. Thanks again, @xjing76!