Mathematics.

TODO: port to latex, or at least markdown + mathjax

This commit actually leaves us in pretty good stead:
 we have a derivation of the formulas used, giving why they are the proper choice
 for use in a gibbs sampler

It doesn't tell us why we should prefer a gibbs sampler over a simple multinormal sampler,
nor how to use. I suppose I could read the Everson & Morris paper.