Filter out non-positive definite Hessians before Laplace approximation

We do not check whether the normal approximation underlying the Laplace approximation to the marginal data density makes any sense given the Hessian at hand. That results in wrong output, because the log determinant will be complex and Matlab only prints the real-valued parts. See https://forum.dynare.org/t/hessian-matrix-at-the-mode-is-not-positive-definite-but-the-marginal-density-is-displayed-anyway/14510/3

Proposed solution: skip the computation of the Hessian is not positive semi-definite and return NaN.