create basin-assembling script

[corviday]

This script should accept a shapefile containining various watershed polygons and a CSV indicating which watershed is part of which basin, and output a shapefile containing the basins, each basin consisting of the union of all associated watershed polygons.

Possibly holes smaller than a certain size in each basin shoudl be removed. I don't think we can require every basin to be a single polygon - "coastal" is a basin - but simplifying them somewhat seems useful.