Let the timings run while I slept;

results: Rcpp2, with hand-rolled loops, is very slightly better than RcppEigen
        (probably every as<Map<MatrixXd> >() is doing a memcpy which is slowing us down)
        extremelynaive.RcppEigen is drastically faster than the best R implementation.
TODO: what I am doing today.
Merge https://github.com/kousu/stat440
Have a bug with the MultivariableRegression test.
It's failing when trying to take the inverse of kappa in rMNIW. I can't figure out why it's failing and would like you to have a quick look to see if you know.