`PintMetaIndex`

[keewis]

Towards #1

With the index refactor coming along nicely, we should start looking into what might be needed to implement a PintMetaIndex to get the missing indexing functionality (sel/drop_sel/reindex/...) to work without the current wrapper methods.

Of course, there's no public API yet, and the general MetaIndex base class is still missing (xref pydata/xarray#6293), but we can still start playing around with this part of xarray – see xarray-custom-indexes for an early example of a custom index. That would allow us to provide feedback on how that MetaIndex could be defined / implemented.