DataTree backend

[keewis]

Once #10 is complete, we should be able to expose a DataTree reader function (open_alos2) and a DataTree backend, once that exists (xr.open_datatree)

[keewis]

xarray's version of open_datatree has the backend mechanism, so we should be able to just define a BackendEntrypoint. That would allow us to do this:

xr.open_datatree(path, engine="ceos-alos2", chunks={}, backend_kwargs={"records_per_chunk": 4096})

(and possibly the same with open_dataset, though not sure if that makes sense)