List of useful xarray functions

[TomNicholas]

Only a small subset of xarray methods/functions will actually work on virtual datasets (basically those which only manipulate metadata). It would probably be helpful to explicitly list the ones which do work in the documentation somewhere, so that users can better understand what about their virtual references they can freely change.

In fact the list of methods which would work is mostly the same as the sub-list Dataset Contents in the main xarray docs.

Perhaps such a section should also more explicitly discuss why you cannot reduce or rechunk the virtual datasets.