Allow fmin to be given a simplex, not just a single sample point.

The existing behavior of fmin means there is no way to give it its initial simplex. If you know a priori roughly the scale of your solution or where it is, you may want to give the initial simplex so Nelder–Mead doesn't have to spend function evaluations just expanding the simplex.

This allows x0.shape to be (N+1,N).