Migrate ITreeRegion table to have one row per region

Before the data migration, each i-Tree region had many rows in the ITreeRegion table. Each row's multipolygon contained one polygon.
After the data migration, each i-Tree region has one row in the ITreeRegion table. Each row's multipolygon contains many polygons.

Note that (for me) running the data migration causes some "Message too long" errors to be printed, apparently harmless.

Also, we now declare that i-Tree region codes must be unique in the ITreeRegion table