cartopy: Minimizing incompatibility between cartopy and shapely

Cartopy has a module that both imports Shapely and adds new GEOS bindings: https://github.com/SciTools/cartopy/blob/master/lib/cartopy/trace.pyx. I think this is an unsound approach that can lead innocent cartopy users into DLL Hell.

I’d like to propose two better approaches.

Move all of cartopy’s GEOS dependency into Shapely

The extension code in trace.pyx is rather small and could be moved to Shapely and deleted from cartopy. The DLL Hellmouth would be closed and cartopy users could benefit from binary Shapely wheels. There’s still time to do this before Shapely 1.6.0.

Vendorize Shapely

Cartopy could copy the Shapely source and add build steps for it into setup.py, removing it from the list of dependencies that would be resolved by pip or setuptools. The internal copy of Shapely would be built using the same geos_c as cartopy’s trace module and the DLL Hellmouth would thus be closed.

@QuLogic @pelson I see in other tickets here that conda is what you’re focusing on. Neither of my proposed approaches are need for conda, though moving the extension code into Shapely will have the benefit of removing calls into Shapely’s private internal API and thus making cartopy more forward compatible with Shapely. I think that’s probably the better approach for making pip install cartopy work on OS X.