 dVdX] = gp2d(gpmodel, m, s)
%
% *Input arguments:*
%
%   gpmodel    GP model struct
%     hyp      log-hyper-parameters                                  [D+2 x  E ]
%     inputs   training inputs                                       [ n  x  D ]
%     targets  training targets                                      [ n  x  E ]
%     nigp     (optional) individual noise variance terms            [ n  x  E ]
%   m          mean of the test distribution                         [ D  x  1 ]
%   s          covaria