ify |lagrhess()| callback instead of
% |objhess()| and |conhess()| which returns the hessian of the Lagrangian
%
%     ddL = hess F(x,Y) + sum_{k=1}^{NgNLN} v(k) * hess g_k(x,Y)
%
% as a (possibly sparse) matrix  of dimensions |(Nx+NYnnz) x (Nx+NYnnz)|
% or empty matrix |[]|. If all three callbacks are defined, |lagrhess()|
% is used.
%
%     function [ddL, userdata] = lagrhess(x,Y,v,userdata)
% 
% Finally, the last four callbacks serve to evaluate |NANLN| nonlinear
% and |NALIN| linear matrix c