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.

Also:
* We now declare that i-Tree region codes must be unique in the ITreeRegion table
* For me, running the data migration causes some "Message too long" errors to be printed, apparently harmless.
* Timing eco benefits computation on 28000 trees in a Philly-area treemap, this change made no appreciable difference.
Make unit tests work with a populated ITreeRegion table

* Some tests assumed the table was empty -- fixed to work with a populated table
* Some tests truncated the table -- fixed to not disturb it
Merge pull request #936 from RickMohr/per-instance-itree-codes

Migrate ITreeRegion table to have one row per region