Support `xarray.DataTree`

[forman]

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

The new data format for the Copernicus Sentinel satellite fleet will be Zarr utilizing Zarr groups and sub-groups.
XRLint does not yet take into account dataset groups. Groups would also perfectly represent xcube multi-level datasets. See #30.

Describe the solution you'd like

So far, XRLint focused on validation of flat datasets. Allow validating also xarray.DataTree objects originating from xarray.open_datatree.

Additional context

• ESA EOPF Sample Service
• xcube Multi-Level Datasets