Flexible indexes: handle renaming coordinate variables

[benbovy]

We should have some API in xarray.Index to update the index when its corresponding coordinate variables are renamed.

This currently implemented here where the underlying pd.Index name(s) are updated:

xarray/xarray/core/dataset.py

Lines 3299 to 3314 in c5530d5

	def _rename_indexes(self, name_dict, dims_set):
	if self._indexes is None:
	return None
	indexes = {}
	for k, v in self.xindexes.items():
	# TODO: benbovy - flexible indexes: make it compatible with any xarray Index
	index = v.to_pandas_index()
	new_name = name_dict.get(k, k)
	if new_name not in dims_set:
	continue
	if isinstance(index, pd.MultiIndex):
	new_names = [name_dict.get(k, k) for k in index.names]
	indexes[new_name] = PandasMultiIndex(index.rename(names=new_names))
	else:
	indexes[new_name] = PandasIndex(index.rename(new_name))
	return indexes

This logic should be moved into PandasIndex and PandasMultiIndex.

Other, custom indexes might also have internal attributes to update, so we might need formal API for that.