Add Contrast Normalization

[Charles-Schleich]

I am keen to implement a contrast normalization based on this wikipedia article: https://en.wikipedia.org/wiki/Normalization_(image_processing)

imageproc::contrast seems like the right place to put the functions for linear and non-linear normalization.

I will work on this over the next couple of days.

[theotherphil]

Implemented in https://github.com/image-rs/imageproc/pull/587