BioPAX Xref for non-pubmed articles.

[jvwong]

There is no mechanism in the Biofactoid BioPax templating system for referencing a non-PubMed article.

• see https://www.biopax.org/owldoc/Level3/classes/PublicationXref___-1968381200.html

While we're at it, should include the additional metadata for an article, so downstream consumers don't have to make additional calls/create services for an external server (e.g. crossref).