Delete Logo.h from repository and include minimal PNG decoding library

The ExaHyPE repository has more than 2000 commits and weights only 10MB. Now in January somebody added a 10MB C++ header file containing an uncompressed picture. It's not only possible to read in this picture in any image reader any more, but the repository just blew to the double size and nobody really wants to deal with such files (try to load it in a text editor).

First, the commit introducing this picture should be changed and the monster file deleted out of the repository.

Second, it should be replaced by an actual picture in GIF, JPG, PNG or similar efficient formats (will be less than 20kb!) and a suitable decompression routine should be added to the Demonstrator. This mimics how actual offline initial data generators work. We do not need to introduce a dependency on an external library but can embed an Open Source minimal decoding library. There are a lot of them, for instance one of these:

• https://github.com/nothings/stb/blob/master/stb_image.h - single file header only PNG decompressor (public domain, 200kB code)
• https://github.com/elanthis/upng - another micro PNG decompressor single file library (as-is license)
• http://lodev.org/lodepng/ - another pico PNG decomressor, 500lines (as-is license)
• https://github.com/hidefromkgb/gif_load/blob/master/gif_load.h - a 400 lines GIF reader (public domain, 20kB code)

This is really the way to go as it allows users to exchange the picture with another one at runtime.

I @sven volunteer to implement this.