GeometryType.clazz() & GeoGeometry.headingFromTwoPoints

• new clazz() function on GeometryType to point at the correct Geometry implementation class. Useful for writing custom serializer/deserializer logic for json parsers that don't know what to do with the Geometry interface.
• new GeoGeometry.headingFromTwoPoints function that calculates the compass direction a line between two points is headed.