Removed cartopy dependency and added module-specific logger

Plotting functions previously used cartopy for generating tick labels and as an axis transform. Lat/lon plots relying on cartopy have been rewritten to remove the dependency, because cartopy cannot currently be installed with pip due to a reliance on additional programs (Proj and GEOS) that cannot be installed with pip.

Output messages for the user are now handled with a logger that can be accessed and configured with logging.getLogger('MoonMag'), allowing importing packages to adjust the MoonMag logging level.