Get citation from metadataSource:
Extracts the citation annotation from the metadata annotation of the
object, and returns its value.
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
annotations. (Note that these may be given with any prefix in the metadata
so long as they expand to the same full property URIs.)