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.
Why would it suffer too much? I don't think Correlation with weights == nil should return a different result from when all of the weights == 1. It looks like the compensation corrects for some numerical noise? The composition should equal approximately 0. 

This will need a comment.