Add checks on keys/ids

[lwjohnst86]

... formalise / express more precisely these requirements somehow. Based on how I understand it now:

• We allow resources with no FK (let’s call these main resources) or one FK (let’s call these dependent resources).
• Main resources must have the observational unit ID in their PK.
• In dependent resources, the FK must point to a main resource.

Originally posted by @martonvago in #151 (comment)