Psp improvements
Several improvements in the psp reader. The main ones being 16 bit integer support and support for indexed and grayscale images.
I would appreciate if someone could test the three 16 bit examples in issue #49 to make sure I didn't mess up the endianness conversions.
Examples for the indexed and grayscale images can be found here: https://github.com/mike-lischke/GraphicEx/tree/master/TestSuite%20original/PSP in the 16 colors, 256 colors and Black and White folders.