Add methods for coda

[nk027]

It makes sense to work on the interface to coda and facilitate it's use.

[nk027]

It makes sense to provide a method for coda::mcmc, that (at the very least) adds the hyperparameters. This may include coda::mcmc.list for multiple chains (including a check whether the chains are comparable).

Other possible methods for:
mcpar, nchain, niter, and nvar.

We should also look into adding betas.

[nk027]

Looks very nice so far!

Betas are now also added. I've also moved the data preparation (which is the same across as.mcmc, density and plot to its own function in 15_prep_data.R. I think the other methods can be put off for now, as they're available after converting to coda.

[nk027]

Okay note that there's some considerations regarding the use of coda.

We provide two methods for as.mcmc(), but do not really want to import+export it. Before we bypassed that by exporting the methods as (weird) functions, which seemed to work. Now there seems to be an issue with that under Windows, which surfaced when testing the vignette. Apparently the functions are not always recognised as methods, which means that as.mcmc produces some abomination out 'bvar' objects.

It seems like this can be bypassed / fixed in a reasonable way by dynamically registering these functions - i.e. if coda is loaded, or as soon as it is. This is done in zzz.R and was inspired by the hms package.