updated docstring, fixed motion blur

[Hvitgar]

list of changes

• updated docstring to be in accordance with the changes made to the corrupt() function (resolves documentation of corruption function #9)
• invalid arguments will now lead to exceptions. Especially, setting severity to invalid values (fixes severity 0 has more than no effect #8) and providing images of wrong datatype will raise exceptions
• grayscale images of dimension (height x width) or (height x width x 1) will be converted to (height x width x 3) internally. Input images with dimensions other than these three options will raise an exception (fixes motion blur fails on grayscale inputs #6)
• motion blur works now on images with small image dimensions (fixes motion blur fails on small input sizes #5). Note: images smaller than 32x32 pixels will still trigger errors in some corruptions, however it is questionable how usefull the corruptions are on very small images. Therefore, currently the input size will be limited to 32x32 pixels and the documentation will be updated accordingly