identifier (http://purl.org/dc/terms/identifier) @return [String] DOI, ISBN, URI, etc refering to the reference. This can be repeated in multiple rows to include multiple identifiers, e.g. a DOI and a URL pointing to a pdf of the article. Example: doi:10.1038/ng0609-637; http://www.nature.com/ng/journal/v41/n6/pdf/ng0609-637.pdf