Custom GeoGrouping on Census geography for choropleth maps

[JavadocMD]

Now that the API lets us apply arbitrary groupings to geo selections, it would be nice if those worked when drawing choropleth maps.

At the moment, the mechanism is that a geo strategy is converted to a scope, and then we load geography for that scope. The problem is that a custom GeoGrouping (which is not a CensusGrouping) will result in a CustomScope and the maps tool gives up trying to render CustomScope maps.

This is solvable though -- one could effectively produce new geography which is the union of all of the polygons from the source geography in each group. But this will take some work of course.