Deprecate basemap

[dnerini]

Remaining open question:
Should we remove option to use the deprecated basemap module and leave cartopy as unique option?

For me it looks like there is no reason to use basemap anymore. In 0.18.0b1, they finally added a feature that was missing from cartopy. Namely, lon-lat labeling with different projections (SciTools/cartopy#1117).

Originally posted by @pulkkins in #177 (comment)