Use metadata_column_to_perfdata in ExtraP

Currently Extra-P uses a manual lookup in the metadataframe to get the "p" (x-values) for its modeling. We could optimize this by calling the metadata_column_to_perfdata function before to have all of the data we need in the ensembleframe.

Start by locating ExtraP and its current manual metadataframe lookup for the modeling "p" values. Review metadata_column_to_perfdata and update the flow so the needed data is available in the ensembleframe; done means ExtraP uses that function instead of the manual lookup.

Written by the indexing model from the issue text.