Enable selection of source and target channels from dataset

[clessig]

Is your feature request related to a problem? Please describe.

Currently, it is not possible to select source and target channels from a dataset, i.e. to define forcing, prognostic and diagnostic variables/fields from it.

Describe the solution you'd like

We should support:

ERA5 :
  type : anemoi
  filenames : ['aifs-ea-an-oper-0001-mars-o96-1979-2022-6h-v6.zarr']
  source : ['u_', 'v_', '10u', '10v']
  target : ['10u', '10v']

The naming 'source' and 'target' still needs to be discussed. forcing, prognostic and diagnostic seems to complex and directly specifying source and target seems less error prone. source or source_channels are both reasonable although u850 etc is a physical field and only a channel in ML lingo (for obs, this is a perfect fit and the ML and domain science meaning coincide)

Describe alternatives you've considered

No response

Additional context

No response

Organisation

ECMWF

[clessig]

CC @kacpnowak