Specially intended for the Grid regression case.
        Given the computed log likelihood derivates, update the corresponding
        kernel and likelihood gradients.
        Useful for when gradients have been computed a priori.

def update_gradients_direct(self, dL_dVar, dL_dLen):
        """
        Specially intended for the Grid regression case.
        Given the computed log likelihood derivates, update the corresponding
        kernel and likelihood gradients.
        Useful for when gradients have been computed a priori.
        """
        self.variance.gradient = dL_dVar
        self.lengthscale.gradient = dL_dLen