def Hk(self, k, m_pred, P_pred):  
return self.H[:, :, int(self.index[self.H_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.