Identifiers.  Made foreign keys to 4 tables

We had decided in the fall to implement a single unique Identifiers
table that could be used throughout ODM2.  The table had been added,
but never linked to other tables. These edits modified 4 other tables
to allow for that foreign key link.  Also noted a question about the
need to have a Specimen Identifier if the associated SamplingFeature
already had one (i.e. there is already a 1:1 map for every Specimen to
a unique SamplingFeatureID).