Feat/multivariate dimension generic

[YeungOnion]

extending API changes introduced in #209 / #252 for Multinomial and Dirichlet. Also modifies / reintroduces tests for these distributions. Test values were compared to hand calculations, perhaps I should compare to scipy?

[codecov]

Codecov Report

Attention: Patch coverage is 89.09091% with 12 lines in your changes missing coverage. Please review.

Project coverage is 89.70%. Comparing base (4f362c6) to head (4e3f09a).
Report is 1 commits behind head on master.

Files	Patch %	Lines
src/distribution/dirichlet.rs	89.09%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #253      +/-   ##
==========================================
+ Coverage   89.67%   89.70%   +0.03%     
==========================================
  Files          49       49              
  Lines       10989    11031      +42     
==========================================
+ Hits         9854     9895      +41     
- Misses       1135     1136       +1     

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