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.
Sure.  The sxx and sxy corrections are due to some kind of accumulated errors in the sum of squares due to roundoff when compared to the mean I think, and I'm going by 1.7 in the Chan paper, which doesn't have a -1 in the correction.  I suspect that it is because the order of the correction is on the same order as the mean, but the Chan paper sources it from a "private communication" and I can't find a better explanation.