Rationale

After debugging the issue with matplotlib's contour labelling (seen in #13), it is clear that it was designed before there was a strong transform and path capability. For example, it assumes that there is only one polygon per Path (it completely ignores codes), and it also assumes that paths can be transformed by simply transforming the vertices. Because of this I solved the problem by pre-transforming the paths to the axes coordinate system, and splitting any multi-poly Paths. With this in place, the existing contour labelling functionality works reasonably well (there is room for improvement with the label placement, but that would be a matplotlib enhancement).

Implications

• Closes Contour labelling #13 (thirteen! - that was a long time ago)
• Works around an issue in matplotlib. It is feasible to address this directly in matplotlib (by re-writing some of the algorithm, and splitting other parts out into the matplotlib.path module. I haven't done this yet, and don't think I will have the time to do it justice
• Changes the type that comes out of GeoAxes.contour[f] to a cartopy.mpl.contour.GeoContourSet (via ugly polymorphism)
• Users can use matplotlib's click-to-label functionality, which is nearly cool 😄(I get an ugly traceback on exit).