Add support for CmdStanR objects

[jgabry]

It should be pretty easy to add support for the fitted model objects from CmdStanR.

For models fit using MCMC, there are a few options:

• as.shinystan could convert a CmdStanMCMC object fit to stanfit with rstan::read_stan_csv(fit$output_files()) and then just use the shinystan with the stanfit object

• as.shinystan could extract all the needed info from the CmdStanMCMC object and then use the array method for as.shinystan but also specify the argument to add the sampler_diagnostics, the number of warmup draws, etc.

The second option is probably more efficient, but the first option requires less work.

[jgabry]

For anyone who see this right now and wants to use shinystan after using cmdstanr you can do:

stanfit <- rstan::read_stan_csv(fit$output_files())
launch_shinystan(stanfit)

[jgabry]

@VeenDuco I just added initial support for this on master and v3-alpha.

[nikunj410]

Can you add a worked simple working code for illustration?