Implement open_datatree for Backends

[kmuehlbauer]

• xradar version: latest
• Python version: any
• Operating System: any

Description

The xarray.BackendEntryPoint class has a method open_datatree. We should implement our open_"engine"_datatree functions via this method. Then we can open our radar files like xr.open_datatree(filename, engine="odim").

I'll start with ODIM and GAMIC. Integration process should start after #223.

• ODIM
• GAMIC
• cfradial1
• cfradial2 (should work out of the box, only boilerplate code needed)
• rainbow
• sigmet
• furuno/iris
• nexrad_level2
• datamet
• hpl
• metek

[aladinor]

This issue can be related to #171. It would be something like engine=zarr.