Normal3类定义

法线的相关操作都在图中，实现部分还是大家自己练习，照着图写代码就行了。

类声明：

class Normal3 {
public:
	Normal3();
	~Normal3();
	Normal3(ldouble a);
	Normal3(ldouble a, ldouble b, ldouble c);
	Normal3(const Vector3& v);
	Normal3(const Normal3& n);
	Normal3 operator+(const Normal3& n) const;
	Normal3& operator+=(const Normal3& n);
	Vector3 operator+(const Vector3& v) const;
	Normal3 operator*(ldouble a) const;
	Normal3& operator*=(ldouble a);
	Normal3 operator/(ldouble f) const;
	Normal3& operator/=(ldouble f);
	Normal3 operator-() const;
	Normal3& operator=(const Normal3& n);
	void normalize();
	friend Normal3 operator*(ldouble a, const Normal3& n); 
	friend std::ostream& operator<<(std::ostream& os, const Normal3& n);
	friend std::istream& operator>>(std::istream& is, Normal3& n);
	ldouble x, y, z;
};