find downslope coordinate for D8 direction.

def downstream_index(dir_value, i, j, alg='taudem'):
        """find downslope coordinate for D8 direction."""
        assert alg.lower() in FlowModelConst.d8_deltas
        delta = FlowModelConst.d8_deltas.get(alg.lower())
        drow, dcol = delta[int(dir_value)]
        return i + drow, j + dcol