Remove path_to_geos heuristics

[rhattersley]

Ultimately it'd be nicer to remove the heuristic guesswork from path_to_geos by modifying MPL to actually use the CLOSE_POLY vertex code when a path describes a polygon. In the meantime, we can get a chunk of that benefit by removing the heuristics and making GeoAxes.contourf() ensure the resulting paths use CLOSE_POLY.

(Originally raised in #63.)

[cpelley]

@rhattersley my head is not in this space, but after a little reading around, I can't see anything obvious that suggests that this has been done yet? Can you confirm this for me.
If so, I suggest we modify the milestone for this issue to 0.12.0? Can you do the honors @rhattersley /@pelson?

[rhattersley]

I can't see anything obvious that suggests that this has been done yet?

Me neither.

[cpelley]

Thank you for confirming @rhattersley