Negative binomial distribution

[jannisborn]

Dear @dylkot,

First of all, thanks for releasing this codebase, it is very useful.

While adapting the code for my own needs, I was surprised to see there is no sampling from a negative binomial distribution in the codebase. Instead the original Splatter paper is very explicit about that and the R code also mentions this everywhere (https://github.com/search?q=repo%3AOshlack%2Fsplatter%20binomial&type=code)

I'm wondering what causes this mismatch in the current codebase and whether it's a bug or not. I'm aware that Negative Binomials can be written as mixtures of Poission and Gamma distribution which sounds like a good explanation at first glance, but I wanted to ask you directly.

Thanks a lot