hesschek(net, x, t)
hesschek(net, x, t) takes a network data structure net, together
with input and target data matrices x and t, and compares
the evaluation of the Hessian matrix using the function nethess
and using central differences with the function neterr.
nethess, neterrCopyright (c) Christopher M Bishop, Ian T Nabney (1996, 1997)