concat_members does not retain member_id as coordinate

[jbusecke]

Consider this example:

a = xr.DataArray(np.random.rand(4,5)).to_dataset(name='var')
a.attrs['variant_label']='a'

b = xr.DataArray(np.random.rand(4,5)).to_dataset(name='var')
b.attrs['variant_label']='b'

concat_members({'a':a, 'b':b})

this results in a single dataset with a dimension but noot coordinate member_id

concat_members should obviously carry the information about variant_label/member_id.