Use environment variables to define which hemisphere to default to

[mdtanker]

In PR #212 we added support for Greenland and the Arctic, with some datasets, and extensions of the plotting functions to include the projection EPSG 3413 for the arctic region. To do this, we needed to add the parameter hemisphere to many functions throughout the code to tell PolarToolkit whether to expect data in EPSG 3031 or 3413, for the south or north hemispheres, respectively. While this works, it is cumbersome, especially since most research focus on either hemisphere, but typically not both.

To fix this we should suggest users set an environment variable in there OS, such as polartoolkit_hemisphere, and all our functions which use hemisphere, will check if this variable exists, and use it if so.