Make nested rhat more "arvizian"

[OriolAbril]

As of now, rhat_nested has superchain_ids as a required argument. This is fine for the array interface but I think in general we'll want something more arvizian when using xarray objects. Here are two ideas:

• When superchain_ids is None (default) pass data["chain"] as coordinates. Coordinate values don't need to be an arange for chain and I think keeping the superchain_ids info as coordinate values would fit the workflow nicely.
• As there must be the same number chain elements within superchain_ids, this could also come from a 3rd dimension. That is, instead of chain: 6, draw... superchain_ids=[0, 1, 2, 0, 1, 2]the dims of the array could besubchain: 2, chain: 3, draw...which could be indicated assuperchain_ids: str = "subchain"`. The dataarray function can then stack chain and subchain and get the superchain_ids array from the coordinates of the stacked version.