Release for ETS integration

New features:

• Initialize with numerically given initial distribution function as detailed in issue #4
• Numerical model development as described in 4896fb2

First GitHub release

This release is the first version of NORSE published on GitHub. It is a significantly improved version of the code released together with the paper by Stahl et al. [Stahl et al., Comp. Phys. Comm. 212 (2017)].