Planetmagfields

This release constitutes adding of new features:

- Support for multiple field models for each planet
- Unit testing
- An interactive Jupyter notebook
- Full documentation
- Adding the project to PyPI

Routines to easily access information about magnetic fields of planets in our solar system and visualize them in both 2D and 3D.
Features:

- only numpy, scipy and matplotlib needed to visualize 2D Br plots and Lowes spectra
- internal Hammer projection in the absence of cartopy
- supports all projections from cartopy
- writes vts files for 3D visualization
- potential extrapolation
- spherical harmonic filtering

Bugfixes in this version:

- fixed plot titles.

- Bug fix : fixed normalization factor for m!=0, thanks to tgastine for testing and pointing it out.
- Change default contour levels to 20 : this brings out the Earth's surface field really well.
- Switch to a semantic versioning system (major.minor.patch)