Update Publication.json to capture ISBN, arXiv `<setSpec>` header tag(s)

[seasidesparrow]

Two pieces of metadata relevant to the publication the ingested record appears in are

• ISBN, in the case of books where there is one ISBN for the series and a different one for the chapter, and the chapter's ISBN would go into the persistentIDs field.
• The ArXiv <setSpec> tag in the dublin core header. In classic bibcodes prior to 2007, this was used to generate the bibstem (e.g. astro.ph)

I propose to capture both of these within Publication.json, using two new fields: ISBN similar to the ISSN field, and setSpec which can be used to indicate the harvest set id of an OAI-PMH harvester.