Now with more cowbell!

Version 0.19

• Rivers and grayscale_heightmaps are generated as PNG images, filenames no longer have to be provided.
• The grayscale_heightmap is now stored as a proper 16 bit grayscale PNG.
• Modified color elevation routines so that depressions are still represented as land instead of ocean and heights are normalied.
• Speed of generation increased by almost a factor of 3 (due to update to Platec and making heavy use of numpy).
• World-generation is now deterministic, i.e. generation is 100% reproducible.
• Added support for exporting heightmaps using libgdal (see https://www.gdal.org/formats_list.html for possible formats).
• Added the ability to modify temperature and humidity ranges as well as the temperature/precipitation curve.
• Added the ability to generate scatter plots showing temperature and humidity of all terrestrial cells.
• Added small variations to the temperature-map based on basic orbital parameters.
• Added a satellite-like view of the world.
• Added support to save/load worlds in/from HDF5-format.
• Removed support for saving/loading worlds in/from pickle-format.