Extract relators from a single CSV column

[xurizaemon]

Rather than having a single column fixed to Photographer, support column which nests DSV (pht=Marti Friedlander|dpt=Ralph Hotere) inside the CSV.

The migration can then expand this and pass the extracted values as rel_type and target_name to entity_generate for conversion to a target_id.

There are issues with "CSV-in-CSV" but this is a flexible way to extend the existing CSV to support much wider range of relators (without adding a new column and modifying yml for each new rel_type.

WIP @ https://github.com/xurizaemon/migrate_islandora_csv/tree/relators

[xurizaemon]

Currently using a process plugin to handle this extraction, but I think this might be do-able in yaml, hoping to come back to that.