fre pp split-netcdf-wrapper yaml parsing function

[cwhitlock-NOAA]

fre pp split-netcdf-wrapper currently relies on the same parsing function as remap-pp-components, which takes in a component (type in the yamlfile) and a history_source underneath it and grabs the list of variables associated with that history_source.

However, if you look at flow.cylc in fre-workflows - where we intend to use fre pp split-netcdf-wrapper - we aren't cylcing over (type in the yamlfile) during the split-netcdf steps. We're cylcing over and its variants, which are a flat list of all history_sources in the input yamlfile.

This means that split-netcdf-wrapper can't use the same parsing function as remap-pp-components - at least not yet. If we get around to that big refactor of flow.cylc it would probably make sense to make the split-netcdf tasks loop on type as well as history_source, but the appropriate intermediate step is for split-netcdf-wrapper to parse yamlfiles differently than remap-pp-components.

reference yamlfile: https://github.com/NOAA-GFDL/fre-cli/blob/main/fre/app/remap_pp_components/tests/test-data/yaml_ex.yaml