Gibbs sampler for the multivariate normal, yeah yeah yeah!

Its variances come out on the same order of all the others samplers.
factor the test code
wrong implementation, based on the notes.

and at the same time, I've factored the previous implementation to match.
the two versions are surprisingly symmetric.

curiously... it actually seems to produce the right distribution! go figure!
the symmetry in the two versions is probably the clue to turning one into the other!