feat(output): allow selecting variables for GRIB output

[frazane]

Purpose

Allow users to specify a list of variables (a subset of the variables in the state fields) that they want to include in an output GRIB file.

Code changes

• added a variable keyword to the corresponding classes. When not provided, all fields are written to output.
• also made a bugfix on L262 of grib.py so it uses the correct param (based on the original local definitions used when creating the anemoi dataset), e.g. in our case T_2M would need to be used instead of 2t.