Investigate using GEOS and/or Shapely

[gbrener]

PostgresAdapter includes custom parsing code for PostGIS geometries. In recent weeks I have stumbled upon a few bugs in this parsing code, but have not had time to familiarize myself with their causes and fix them. It occurs to me that we should reduce our maintenance burden and future bugs by integrating with GEOS (or a popular Python / Cython wrapper for it, Shapely). The GEOS and Shapely projects appear to support more geometries, are more rigorously tested, have larger userbases and development resources, and seem to perform well.