Add DataTree class for Tree-like Hierarchical Structure

[rtbs-dev]

From datatree library, which might get merged upstream. This would be incredibly beneficial to have, as someone that wants to make dataset interop specs (generator classes), and these dataclasses would fit the bill perfectly once the datatree functionality is possible.

I realize it's a bit early, perhaps, since it's not merged upstream, but I was sitting considering writing something like a typed datatree class generator myself and thought I'd reach out and see if there was interest in having such a thing live here instead.

[etienneschalk]

Hi,

I think this is a good idea indeed.

Static typing for datatrees would be the ultimate "input validation". What really lack when using xarray is validation, auto-suggestions. Imagine opening a NetCDF file or Zarr folder, and being suggested with variable names to access. Would definitely be a huge time-saver