def Ak(self, k, m_pred, P_pred):  
return self.A[:, :, self.index[self.A_time_var_index, k]]<EOL>
function (k, m, P) return Jacobian of measurement function, it is
    passed into p_h.
    k (iteration number), starts at 0
    m: point where Jacobian is evaluated
    P: parameter for Jacobian, usually covariance matrix.