Consider removing the explicit dependency on cartopy

[marcwie]

Cartopy comes with a lot of external dependencies. Thus installing sedac-gpw-parser using setup.py requires some prior manual work by users such as installing libgeos-dev or libproj-dev. Since the core functionality of sedac-gpw-parser lies in parsing and storing the population data for later use, the depency on cartopy could be removed for the sake of easier installation.

If users then import plot.py they should be greated with a warning asking them to install cartopy manually.