Make foreign keys not nullable

[rod-glover]

A foreign key can be nullable or not, according to whether it is optional or required, respectively.

It appears that all foreign keys declared in all tables in CRMP are nullable. This does not seem logically correct in most cases; a meta_history record with a null station_id does not make any sense. Possible exceptions are objects that are not "owned" by a network; however, at the time of writing, no such objects exist.

TASKS:

• Determine in what cases NULL FKs are logically allowed, i.e., in what cases it is optional.
• In cases where FKs are logically required (most cases), add the NOT NULL constraint.