Extracts the citation annotation from the metadata annotation of thenexml
object, and returns its value.
get_citation(nexml)a nexml object
the citation if the metadata provides one that is non-empty, and NA otherwise. If multiple non-empty annotations are found, only the first one is returned.
Currently the implementation looks for dcterms:bibliographicCitation
annotations. (Note that these may be given with any prefix in the metadata
so long as they expand to the same full property URIs.)