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 understand what happened now. Sxy has an implicit 1/(N-1) term, and sxx and syy each have an implicit 1/(sqrt(N-1)) term, so the two cancel. Could you change the tex comment to say what this is doing, namely

sum_i {w_i (x_i - meanX) * (y_i - meanY)} / sqrt{\sum_i{w_i * (x_i - \mu_x)^2 } * \sum_i{w_i *( y_i - \mu_y)^2}}

The formula is more complicated, but the code that follows is more clear.