Update xarray guidance to use datatree where applicable

[asteiker]

The latest xarray release introduces xarray.datatree into the library. This can simplify our guidance when opening NSIDC DAAC hierarchical HDF data, as you no longer need to point to a specific dataset initially. We ought to consider whether any of our tutorial can be updated to utilize datatree.

[andypbarrett]

Adding this as a Cookbook issue.

Should we close this issue for Tutorials.