Use clone when creating a geom from a geom

If a geometry is instantiated from another geometry of the same type, just
call clone. The logic is implemented in the geos_*_from_py functions, both in
the speedups and in the Python implementations.
Make sure to test Polygon created from Polygon
Use getCoordinateDimension

I'm pretty sure this is the intent of this code.
Merge pull request #165 from jwass/geom_clone

Geom clone