Generalized Poisson distribution

The generalized poisson distribution has some potential when it comes to the estimation of underdispersed count data and is much more efficient to evaluate the the conway-maxwell poisson distribution. It may have trouble due to its complex support and it remains to be evaluated whether this distribution is robust enough to be implemented as a native family in brms. More information about the generalized poisson distribution may for instance be found at https://rdrr.io/cran/VGAM/man/genpoisson.html.