Signatures of Point3D sub and mul does not match behavior of Point2D

Point2D and Point3D should have the same behavior in use and in method signatures. For example, Point2D's sub method does a minus-equals on the coordinates, whereas Point3D's sub method returns a new object of the subtracted result:

public void sub(Point2D other) { x -= other.x; y -= other.y; }
public Point3D sub(Point3D other) { return new Point3D(x - other.x, y - other.y, z - other.z); }

This could lead to mistakes when coding against the api.

首先定位 Point2D 和 Point3D，并比较 sub 和 mul 的声明与实现。检查现有调用方或测试，了解它们在变异与返回值方面的行为。完成的标准是两个点类型具有匹配的签名和一致的行为，并且已根据现有用法验证预期的 API 选择。

由索引模型根据 Issue 内容生成。