severity 0 has more than no effect

[ojwenzel]

When applying image corruption with severity 0 (calling corruption()) (for example in an iterative loop), the resulting image is actually corrupted quite strongly. Maybe, either do not allow zero (as its call might be questionable), map zero to zero effect or document the meaning of zero severity.

[Hvitgar]

Thank you for pointing out. This is due to python's array indexing magic, setting severity to 0 will actually result in corruptions of severity 5. I will add a check to make sure that only valid values can be passed to the function