added cartopy installation instructions
because some developers might not have cartopy ...
The UCSC GitLab service will temporarily be unavailable today, Thursday, March 13th starting at 3:30 PM for unplanned system maintenance while a necessary software upgrade is underway. The service will be unreachable until this work has been completed. If you need to submit coding assignments, make commits, then push your working tree once service has been restored.
because some developers might not have cartopy ...