Support DataTree in xarray.concat

[shoyer]

What is your issue?

Something like this works, but is a little awkward -- especially the skipping over dimensions that are only defined on some node levels:

def concat(*datasets, dim):
    if not all(dim in ds.dims for ds in datasets):
        return None
    return xarray.concat(*datasets, dim=dim)

xarray.map_over_datasets(concat, tree1, tree)

[TomNicholas]

I agree - but I think this issue is a duplicate of #9106.

[TomNicholas]

Gonna keep this open as a small sub-issue that's part of the checklist in #9106