Add foreign key relationships to MCH database schema

[isedwards]

For historic reasons, the MCH database schema does not implement foreign key relationships. The business logic is embedded into the Free Pascal software.

In order for the OpenCDMS abstraction layer to fully understand the relationships, we need the information equivalent to which fields represent foreign keys to other tables.

(This task was discussed in PAT meetings as MCH contribution to data model development in 2021)