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.
I believe that cancels out as well, because the same sample size and weights are used in the calculations of sxy, syy, and sxx.