braindump matrix-normal facts
document the quirk of diag()
I've discovered useful Rcpp linear algebra libraries; this commit links them
The magic trick to lookup how stuff is done in R-core
More future TODOs
finding packages left right and center
More of the Rcpp naive algorothm fleshed out

This also includes some experiments with Eigen (which can be removed at leisure)
oh. durp. I don't need Eigen to store my return values, only to do the linear algebra heavy lifting
invert Psi; okay, I'm getting the hang of this
more hard fought lines of the algorithm
Naive and Snappy algorithms written in Eigen.

This commit is large, going against my usual habit, but by the time I was halfway done it was too broken to go back and start again.
Mainline the Eigen code and fix a critical bug in snappy1

Critical bug was this: the factoring was happening inside the loop. D'oh! Now snappy1 is indistinguishable from the others.
correct the RcppEigen naive algorithm so that it actually uses kappa

also add header comments and remove the stray debug returns which would be caught by rNIW.typechecked()
repair rmvnorm, which was using the wrong sided chol()

Funny, the naive algorithm was the incorrect one this time! And the bug wasn't

Also, export rmvnorm while we're at it, because it's useful (but warning, will shadow MASS's version!! if things suddenly start crashing after this commit, you'll know where to look)
match rmvnorm to MASS's mvrnorm (which is misspelled)