Update to cartopy

[kadrlica]

Basemap is now deprecated in favor of cartopy.

[kadrlica]

I've started to looked into this since support for basemap is officially vanishing in 2020 along with the deprecation of Python 2. Some issues (of widely varying severity):

• The structure of cartopy is quite different, returning various objects that are passed into matplotlib axis. This will require some fairly major restructuring.
• Development of cartopy seems a bit scattered. It is unclear how stable it is.
• Cartopy doesn't have the McBryde-Thomas projection that we've come to know and love in DES.