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.
Correct me if I'm wrong, but it seems like MeanVariance is sufficient to for a benchmark. The rest is trivial after that, and just makes running the benchmarks take longer.