Make `cartopy` requirement optional for installs.

[brews]

Right now, baysparpy depends on cartopy. Cartopy is needed to plot the gridpoints (analogmap()) output from an analog prediction (predict_seatemp_analog()). Other components in the package do not need cartopy, just this one plotting function.

Cartopy can be a pain to install, especially for new users.

Might want to make cartopy optional for installs, note this in the documentation and print out an error message on import saying that cartopy needs to be installed... something like this.

[brews]

Be sure to declare it as an extra requires in setup.py (https://setuptools.readthedocs.io/en/latest/setuptools.html#declaring-dependencies)

[brews]

I'm removing this from the v0.0.2 milestone because I'm uncertain about this. The package is so large that we are most likely going to distribute this through conda anyways. Wait until we get more user feedback and see if the required cartopy dependency is really a problem.