Possible PR: making posterior draws compatible with MCMC diagnostic packages

[noamross]

I've used the following function in a couple of projects, which runs gam.mh in parallel and manipulates the outputs into a structure of the form that the {posterior}, {coda}, {bayesplot}, and other packages that do MCMC manipulation expect. This makes it easier to get MCMC diagnostics and diagnostic plots from gam.mh outputs.

https://github.com/ecohealthalliance/sabrenet-rousettus-dynamics/blob/b45571ef86cd51b7f19177c30fe04192dcaca917/R/fit_multinomial_model.R#L88-L121

I wonder if you would be interested in a PR that incorporates some of this into post_draws. I think it would (a) allow for multiple parallel draws, and (b) either have an argument to modify the output format, or a function to convert the posterior draws into the standard tidy table or array that can be used for MCMC diagnostics.