rainymotion v0.1

The current release v0.1 of the rainymotion library supports the following paper in the GMD Discussions:
Ayzel, G., Heistermann, M., and Winterrath, T.: Optical flow models as an open benchmark for radar-based precipitation nowcasting (rainymotion v0.1), Geosci. Model Dev. Discuss., https://doi.org/10.5194/gmd-2018-166, in review, 2018.