add support for posterior package

[strengejacke]

Description

This PR aims at adding this feature...

Proposed Changes

I changed the foo function so that ...

[strengejacke]

@DominiqueMakowski you may merge if everything works.

I mostly copied data frame methods to create the .draws methods, so we support the posterior package now.

[mattansb]

Looks good to me.

[mjskay]

I followed this over from stan-dev/posterior#216. Very cool to see support for {posterior} here!

I noticed that the .posterior_draws_to_df() helper doesn't have an implementation for the "draws_rvars" format. You should be able to do something like this:

.posterior_draws_to_df.draws_rvars <- function(x) {
  .posterior_draws_to_df(posterior::as_draws_df(x))
}

Alternatively you might replace all of the format-specific implementations of .posterior_draws_to_df() with something like this:

.posterior_draws_to_df <- function(x) {
  datawizard::data_remove(as.data.frame(posterior::as_draws_df(x)), c(".chain", ".iteration", ".draw"))
}

Which would eliminate the need to test/maintain those functions here and make you future-proof to any changes in posterior formats.

[strengejacke]

Thanks @mjskay, I used your latter approach, which indeed seems more stable, also regarding potential future changes.