Ability to georegion code based on a string

This enables directly georegion coding against a column containing zip code strings (eg. "98122") instead of having to geocode the zip code to an additional point column and then georegion coding that column against the region polygon.
The name `MapRegionCache` is a bit confusing vs `SpatialRegionCache`.  After all, aren't maps spatial in nature?  (I know you are referring to a HashMap, maybe call it HashMapCache or something.

Also, missing ScalaDoc.