Mistakes in vector subtraction in kurve geometry.h

[azaparov]

Please note "-" position in following functions:

inline	const	Vector2d& operator-=(const Vector2d &v){dx -= v.dx; dy =- v.dy; return *this;}		
const	Vector3d& operator-=(const Vector3d &v){
			dx -= v.dx; dy =- v.dy; dz = -v.dz; return *this;}			// v1 -= v0;

[J-Dunn]

This was fixed at some point. Well spotted. Someone should close.