Stable LEEM visualization

The functions that are imported in agfalta.leem (see agfalta/leem/__init__.py) are stable and working. They are documented in the jupyter notebook under example/LEEM_plotting.ipynb. Also, this version is now backwards compatible with notebooks created with versions before 0.2.0. However, it is recommended to change the import statements into from agfalta import leem and use the functions and classes provided there, instead of importing agfalta.leem.plotting.