Establish multiple rescaling options

[tretherington]

With the exception of the Perlin noise function, all functions currently rescale values between 0 and 1 linearly. The Perlin noise function also has options to use absolute and squared functions before linear rescaling to provide different effects. It could be useful to make these other rescaling options into general functions, and allow other NLM functions such as the mid-point displacement to also call them - as like Perlin noise the mid-point displacement algorithm produces negative and positive values.