Allow variable type to be overridden. #29.

MDES 3.3 has a table with several PKs according to the VDR schema
(link_child_partic_org). Upon inspection, it is plain that the extras
are supposed to be FKs.

This feature allows overriding the named type provided for a variable
in the schema to allow switching these to be FKs.
Add TransmissionTable#primary_key_variables. Done with #29.

We assume in many places that MDES tables have a single PK. This
method allows us to verify that that is true for each new spec
version. (MDES 3.3 gave us a scare, but it looks like in that case it
is just a schema error.)

Extracted from the warehouse.
Add MDES 3.3 support. Closes #29.