Add support for rvars draws format

[avehtari]

To avoid need for manually switching between different draws formats, it would be nice if bayesplot functions would allow draws_rvars format as input. At least mcmc_hist already works with draws_array, draws_matrix and draws_df.

[jgabry]

Yeah I agree. I haven't had much time for bayesplot lately but I'd like to find the time to do a release. There are a bunch of open pull requests and some easy issues like this one that could go in a new release.

[jgabry]

@Ozan147 the function that I think needs to be modified to support the posterior formats is prepare_mcmc_array():

bayesplot/R/helpers-mcmc.R

Line 8 in 60edc5a

prepare_mcmc_array <- function(x,

[jgabry]

@avehtari I just merged @Ozan147's pull request (thanks Ozan!) so the master branch should now support all the posterior draws formats for the MCMC plots.

[avehtari]

Great! Thanks @Ozan147 !