Interface for - basically 3D - noise generation algorithms, based on the
book: Texturing & Modeling - A Procedural Approach
The main concept is to look at noise as a basis for generating fractals.
FractalSum is the simplest form of fractal functions summing up a few octaves
of the noise value with an ever decreasing (0 < roughness < 1) amplitude
lacunarity = 2.0f is the classical octave distance
Note: though noise basis functions are generally designed to return value
between -1..1, there sum can easily be made to extend out of this range.