negbin now works with weights.

[JockLawrie]

negbin with weights is currently broken.
This branch fixes this, so that this works for example: negbin(F, D, args...; wts=wts).
A test case is included.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.04%. Comparing base (e2f6c98) to head (e079c8b).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #556      +/-   ##
==========================================
+ Coverage   89.86%   90.04%   +0.17%     
==========================================
  Files           8        8              
  Lines        1125     1125              
==========================================
+ Hits         1011     1013       +2     
+ Misses        114      112       -2     

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