Add foreign keys from maps to macrostrat

[davenquinn]

There are no foreign keys from maps to macrostrat, even though data in the maps schema (especially legend lookup tables, etc.) requires strong links to the Macrostrat column dataset to function.

• These foreign keys should generally get ON DELETE SET NULL or similar
• They may interfere with dumping and loading development versions of the Macrostrat schema
• This is lower priority than foreign keys within the Macrostrat schema proper, which have their own complexities

[davenquinn]

This requires #78 to be solved