Add Distributions to the Random generation

[joneubank]

• Add distribution as an optional parameter to all the generators.
• Add as an optional parameter to context generation, so that it can be the default distribution for everything called within that context
• Make the default distribution added to a context be Uniform

Interface for this something like:

type Distribution = (x: number) => number;

example:

const Uniform: Distribution = (x) => x;

or

const Sinusoidal = (periods: number, offset: number):Distribution => (x) => Math.sin(x*PI*periods+offset*TWO_PI);