Add convergence methods for draws objects

Currently the diagnostic functions rhat, ess_tail and ess_bulk are intended to be used on a single variable matrix or rvar, e.g. as part of summarise_draws. However, they can be applied (incorrectly) to a draws object (all work on draws_array, some work on draws_df too), and then return a single numeric value which is not meaningful.

In contrast rstar takes a multivariate draws object as input and returns an interpretable value. As rhat and rstar are somewhat similar in interpretation, the different expected inputs could be confusing.

Although it is specified in the documentation that these functions expect certain input, maybe it is worthwhile doing some type checking for these functions and returning a warning / error when used incorrectly (especially when used on a draws object) and pointing to summarise_draws.

Example:

ex <- example_draws()
rhat(ex)
# 1.00185 # Not interpretable

rstar(ex)
# 2.066 # Interpretable

Read the implementations and documentation for rhat, ess_tail, ess_bulk, rstar, and summarise_draws, then reproduce the example with example_draws(). Determine the supported input types and the intended warning or error behavior, and add tests covering draws inputs while confirming summarise_draws remains the suggested path.