Improve handling of duplicate `param` and `variable` metadata keys from NetCDF data

[aaron-hopkinson]

NetCDF data has both param and variable keys in the metadata – there are currently workarounds in the code to handle this as a special case when grouping fields (e.g. PR #182).

This issue covers handling these keys in a more consistent way (either here, anemoi-datasets or earthkit-data, as appropriate).