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

[andersy005]

A few weeks ago, we experimented with Collection, which provides a high-level mapping of name/key to Xarray.Datasets. The xcollection.Collection class is a dict-like class. It subclasses collections.abc.MutableMapping and provides additional functionality like

• .map(): applies a function to each dataset in the collection.
• .choose(): returns a collection with datasets containing all or any of the specified data variables.

Seems it would be convenient/neat to use this Collection in the .to_dataset_dict() method i.e. return a Collection object instead of a regular dictionary of datasets. Usage of this object is likely going to be experimental until https://github.com/TomNicholas/datatree is stable and ready for public usage.