一个3D矢量类CVector3D，含三种构造函数，并有拷贝，点积，叉积，求模，单位化 等操作，并实现”+ - = == += -= * [ ]” 等运算符的重载。（用C++实现） -A 3D vector class CVector3D, with three constructors, and copy, dot product, cross product, the modular unit of the other operations, and to achieve " +