ance by inputs         [D*E x n*D]
%   dMdt      output mean by targets                                 [ E  x n*E]
%   dSdt      output covariance by targets                           [E*E x n*E]
%   dVdt      inv(s) times input-output covariance by targets        [D*E x n*E]
%   dMdX      output mean by hyperparameters                         [ E  x P*E]
%   dSdX      output covariance by hyperparameters                   [E*E x P*E]
%   dVdX      inv(s) times input-output covariance by hyper-par.     [D*