Abstract progress bars

[njtierney]

I think it might be possible to turn bits of code like:

        if (verbose) {
          pb_warmup <- create_progress_bar(
            phase = "warmup",
            iter = c(self$warmup, n_samples),
            pb_update = pb_update,
            width = self$pb_width
          )

          iterate_progress_bar(
            pb = pb_warmup,
            it = 0,
            rejects = 0,
            chains = self$n_chains,
            file = self$pb_file
          )
        } else {

into something like

pb_warmup <- create_progress_bar_warmup()
iterate_progress_bar(pb_warmup, 0, 0)

Perhaps by putting a couple of objects into the slots of R6 objects? It's a small change, but it happens in a couple of places and it might just be another small thing to improve clarity when viewing the code