Fix to_index(): return multiindex level as single index

[benbovy]

• Closes groupby(multi-index level) not working correctly on a multi-indexed DataArray or DataSet #6836
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst

[dcherian]

I like this fix in general: we fix the backwards incompatible change and work to remove MultiIndex special cases separately.

Though this PR seems to have broken a lot of things :)

[benbovy]

Yes I agree it would be nice if we can roll back this breaking change. However, it really conflicts with .xindexes that returns the same index instance for each of its corresponding coordinate. This roll back seems to mostly break things where we need to be smart while handling multi-index coordinates passed to DataArray / Dataset constructors. This might be tricky to solve. It would probably be easier to do it after #6392.

[benbovy]

Tests should be ok now, although this is not a super clean workaround. IndexVariable still needs some more refactoring anyway.

[dcherian]

Thanks @benbovy