撤单

        Arguments:
            amount {int} -- 撤单数量

def cancel(self):
        """撤单

        Arguments:
            amount {int} -- 撤单数量
        """

        self.cancel_amount = self.amount - self.trade_amount
        if self.trade_amount == 0:
            # 未交易  直接订单全撤
            self._status = ORDER_STATUS.CANCEL_ALL
        else:
            # 部分交易 剩余订单全撤
            self._status = ORDER_STATUS.CANCEL_PART