Simplify sigs

As discussed at https://groups.google.com/forum/#!topic/gonum-dev/nOWoTO2ilVM

This is more of a request for comment than something I want to merge right away.  In particular, I've changed some of the algorithms to use a correction.
Well, with perfect precision, the compensations are 0, right?. The first Covariance test matches matlab and wolfram alpha, so it must be correct, but I'm not happy about having an implementation we can't explain.