opening assets that contain multiple groups

[keewis]

The stac backend currently implements open_dataset, which is useful for data that maps to a single Dataset object.

However, often satellite datasets map to a multiple groups (see e.g. the EOPF sample service's "product" asset), so it would be useful to also support open_datatree. This would even allow us to add back support for opening ItemCollections / Items (provided we can figure out the right assets using the asset's role attribute).

(I volunteer to draft an implementation if this sounds good to you)

[jsignell]

Yeah I like the idea of implementing open_datatree. If you have time to work on it that sounds great!