Loading all groups from a netCDF file

[TomNicholas]

We want to be able to load all groups from a netCDF file as nodes in a DataTree, simply via

dt = open_datatree('some_data.nc')

To populate this tree we need to know the structure of all the groups in the netCDF file, but currently in xarray the open_dataset function makes it pretty difficult to see this information. Instead it only allows you to open one group (whose name you have to know in advance!) via the group keyword argument.