Smooth delta functions to compute Jacobian and Hessian from samples

[stevengj]

@mleprovost

I forgot to mention that the samples have a relative small spread about the mean, so we are only interested in a very local behavior of the function.

If the function G isn’t changing much among your samples, so that it is well approximated by a low-order polynomial, then I would just try a polynomial or radial-basis-function fit.