support datatree #171
Description
It would be great if xdggs offered first-class support for xarray.DataTree so we can:
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Select regions by lat/lon across all datasets in a DataTree (e.g., apply dggs.sel_latlon to every leaf node in one call), and
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Interactively visualize multiple DataTree nodes together on a single map (e.g., overlay different variables/datasets) via a DataTree-aware .explore() or similar API.