Questions about order modification

Hi Laffini. When examining the Order class code in the core, I noticed that the side attribute was not marked as final. Upon further investigation, I found that the change order interface provides the ability to modify the side. However, generally speaking, exchanges do not allow modifications to the side. Therefore, I am somewhat puzzled about this. @Laffini

首先阅读 core 中的 Order 类，以及 issue 中提到的变更订单接口。确定对于这个 matching engine，改变 side 是否是预期行为；如果不是，则定义所需的设计变更及其完成标准。目前该 issue 是一个问题，没有指定任何测试。