Support for groups

[TomNicholas]

We should support generating references from files containing multiple groups in the same way that xr.open_dataset and datatree.open_datatree work.

So we should add a new open_virtual_datatree function, and a new (optional) group kwarg to open_virtual_dataset.

This can be done right now using the datatree package (as an optional dependency imported inside open_virtual_datatree) but once that gets merged into xarray main (which is happening right now) we can get rid of that dependency.

See #78 (comment) and #11.

cc @sharkinsspatial

[TomNicholas]

One thing I realized about this is that concatenating multiple DataTree objects is currently a little awkward. I don't know if this is actually a common pattern, but imagine you had two netCDF files each with groups, and you wanted to concatenate group1 in file1 with group1 in file2 etc.

Adding open_virtual_datatree would allow you to open the files like this:

vdt1 = open_virtual_datatree('file1.nc')
vdt1 = open_virtual_datatree('file2.nc')

but currently you can't do

combined_vdt = xr.concat([vdt1, vdt1], dim='time')

because xr.concat doesn't understand DataTree objects. To get around this you should be able to do

from datatree import map_over_subtree

concat_datatrees = map_over_subtree(xr.concat)

combined_vdt = concat_datatrees([vdt1, vdt1], dim='time')

but it raises the question of whether the xarray DataTree upstream integration should include generalizing xr.concat.