storing superchain_ids for rhat_nested

I have been working a bit with many short chains, and using nested R-hat as a convergence diagnostic. rhat_nested has an additional argument superchain_ids which is a vector specifying which superchain each chain belongs to. My current workflow is to add the superchain_ids as an attribute to a posterior::draws object, and then do something like

summarise_draws(draws, rhat_nested = ~rhat_nested(.x, superchain_ids = attr(draws, "superchain_ids"))

This is a bit cumbersome, and maybe it could be improved with:

• a new function add_superchain_ids, which will then add the superchain_ids to the draws object (as an attribute?)
• have summarise_draws automatically use the stored superchain_ids for rhat_nested

Any thoughts on this? I could make a PR if this seems reasonable

Start by reading rhat_nested and summarise_draws to understand how superchain_ids are currently passed. The issue needs a decision on whether to add an add_superchain_ids function, store the values as an attribute, and have summarise_draws use them automatically. Done means the API and automatic behavior are agreed and implemented consistently.