Test Taxonomic Classifier implementation

Added TaxonomicClassifiers table to ODM2Core schema, and also a
TaxonomicClassifiersExternalIdentifiers cross-reference table in the
ODM2ExternalIdentifiers schema (whew! long table and field names!).
TaxonomicClassifiers could be shortened to Taxa or Classifiers (IEDA
uses Classification), but I think that these do not convey the full
range of uses quite as well.