Use symmetric eigensolver for diagonalizing rate matrix

With my BLAS, and a 500x500 matrix, the hermetian eigensolver is about 5x to 10x faster than the nonsymmetric one.
I don't know. It kind of helps when writing the code though, because the cython compiler will choke if you try to use a python function in the section, so it can help keep straight which is a C function and which is a python function.