Integration with xarray datatree

[andersy005]

Is your feature request related to a problem? Please describe.

A while ago, we added support for an xcollection.Collection object which contains a mapping of datasets. This object behaves like a dictionary with additional functionality. However, with the more recent development of xarray-datatree --which will eventually be integrated into Xarray proper--, we should transition to using DataTree instead of Collection.

• Use Collection object implementation from NCAR/xcollection in intake-esm #382

Describe the solution you'd like

• Provide a cat.to_datatree() method which returns an xarray DataTree object
• Remove the .to_collection() method

Describe alternatives you've considered

We currently provide a cat.to_collection() method which returns an xcollection.Collection object.

Additional context

Add any other context or screenshots about the feature request here.

Cc @mgrover1