Update NWPDataSource with new NWP variable names

[JackKelly]

The new NWP to Zarr conversion script brings in 17 NWP variables (up from 10 in the old Zarr):

['cdcb', 'lcc', 'mcc', 'hcc', 'sde', 'hcct', 'dswrf', 'dlwrf', 'h', 't', 'r', 'dpt', 'vis', 'si10', 'wdir10', 'prmsl', 'prate']

Although, we should do some exploration to make sure we actually want all of these! See the Met Office's documentation: we're consuming and merging "Un-split File 1" and "Un-split File 2" (although the variables available in the grib files doesn't exactly line up with the docs!)

Also change on_premises.yaml to point to the NWP zarr in /mnt/storage_ssd

[JackKelly]

I think I've finished the code changes in PR #387

Still TODO:

• Plot some data from all these new NWP variables to make sure we actually want to use all these channels in our ML models
• Check that prepare_ml_data.py runs
• Compute means and standard deviation for new NWP Zarr #407