Implement addDataUnit and/or other methods for adding concrete DataUnit instances in SqliteRegistry.

This is tricky because it needs to account for how to ensure join tables between DataUnits are updated as well (i.e. should adding a Visit add the overlaps between that Visit and all existing SkyMaps?).

This will probably require some additions to the Registry interface. Adding methods/arguments that depend on specific concrete DataUnits is fine for base Registry/SqlRegistry, but we should try to implement these in terms of more generic methods that must be implemented by new Registry subclasses

Unit tests for this ticket should include inserting all concrete DataUnit types.