[float] 昨日结算价（期货专用）

def prev_settlement(self):
        """
        [float] 昨日结算价（期货专用）
        """
        try:
            return self._data['prev_settlement']
        except (ValueError, KeyError):
            pass

        if self._prev_settlement is None:
            trading_dt = Environment.get_instance().trading_dt
            data_proxy = Environment.get_instance().data_proxy
            self._prev_settlement = data_proxy.get_prev_settlement(self._instrument.order_book_id, trading_dt)
        return self._prev_settlement