Get the XML namespace for a version of EMLSource:
Utility function for use when filling in
vocab in various representations of EML. This is a little more future-proof
than keeping a dictionary for each version since this won't break on the next
eml_ns(version = eml_version())
EML version, currently either eml-2.2.0 (current version) or eml-2.1.1. Defaults to current version.