related_identifiers table

[nathcast]

The table related_identifers is a tricky one: It should be a pivot, relating the metadata_documents, with external or internal documents, but we do not have a table for these documents... Incidentally, that table could be the eraPapers table which is in eradoc at the moment and should be here.

The related identifiers can be inside documents like the medatata documents, or other documents like papers and so on. It seems we need a table that lists all these documents and datasets that are related.

Or have two kinds of related documents: those we maintain here and are related, and those outside that are lined via their DOI from another table.

This table is not DRY!