Allow setting multiple indexes per DataArray / Dataset

[benbovy]

Currently we can only set one index (from a set of coordinates) per DataArray / Dataset object, using .xoak.set_index().

I wonder if there could be cases where we have in the same DataArray / Dataset two or more independent sets of coordinates for which we want to set an index. If yes, we could simply use a mapping of "coordinate set -> index" internally in the accessor, instead of only one index attribute.