returns the full Hessian matrix <CODE>h</CODE> corresponding to
the second derivatives of the negative log posterior distribution,
evaluated for the current weight and bias values as defined by
<CODE>net</CODE>.

<p><CODE>[h, hdata] = mlphess(net, x, t)</CODE> returns both the Hessian matrix
<CODE>h</CODE> and the contribution <CODE>hdata</CODE> arising from the data dependent
term in the Hessian.

<p><CODE>h = mlphess(net, x, t, hdata)</CODE> takes a network data structure
<CODE>net</CODE>, a matrix <CODE>