Short-cut when transforming PlateCarree -> PlateCarree in mpl.
Tweaked performance of PlateCarree->PlateCarree transformations in mpl.
Added 'quick_vertices_transform' method to Projection, and made use of it in the mpl transform.
@rhattersley's review actions.
Final review actions.
Merge pull request #260 from pelson/plate_carree_performance

Short-cut when transforming PlateCarree -> PlateCarree in mpl.