Add relatedItem support

[tmorrell]

Not sure whether we want to implement this in doi2rdm or irdm_harvester. We don't really need to implement this until microPublication has their implementation in production.

DataCite now has relatedItem metadata, which includes information about journals.

If relatedItems is present

If relatedItemType == Journal

Then

relatedItem.titles.title -> custom_fields.journal:journal.title relatedItem.volume -> custom_fields.journal:journal.volume relatedItem.issue -> custom_fields.journal:journal.issue relatedItem.firstPage and relatedItem.lastPage or relatedItem.number -> custom_fields.journal:journal.pages relatedItem.publisher -> metadata.publisher if relatedIten.relatedItemIdentifier.relatedItemIdentifierType == ISSN, then relatedItem.relatedItemIdentifier.relatedItemIdentifier -> custom_fields.journal:journal.issn

[tmorrell]

Example record https://api.datacite.org/dois/10.17912/micropub.biology.001246