add Bell distribution

[wpetry]

This feature request would add support for the Bell distribution. The Bell distribution is suitable for count data, has a single parameter, and can model overdispersed counts.

The PDF is:

$$Pr(Y=k)=\frac{{\alpha }^k{e}^{-e^{\alpha }+1}{B}_k}{k!},k=0,1,2,\dots$$

where $\alpha >0$, and $B_k$ are the Bell numbers (i.e., unique partitions of a set).

The R package fndemarqui/bellreg has a Stan implementation.