Add gamma

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 97.16981% with 3 lines in your changes missing coverage. Please review.

Project coverage is 94.97%. Comparing base (9301d9d) to head (8ca507e). Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
src/distributions/gamma.jl	96.42%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #20 +/- ## ========================================== + Coverage 94.36% 94.97% +0.61% ========================================== Files 13 14 +1 Lines 1508 1592 +84 ========================================== + Hits 1423 1512 +89 + Misses 85 80 -5 ```

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