implement unary negative on vectors

MotionCore/Vector3d.cpp

@@ -158,3 +158,8 @@ void Vector3d::normalize() noexcept
     y *= inv;
     z *= inv;
 }
+
+Vector3d Vector3d::operator-() const noexcept
+{
+    return Vector3d(-x, -y, -z);
+}

MotionCore/Vector3d.h

@@ -24,6 +24,7 @@ public:
     Vector3d normalized() const noexcept;
     void normalize() noexcept;
 
+    Vector3d operator-() const noexcept;
     Vector3d& operator=(const Vector3d& other) noexcept;
     Vector3d& operator=(Vector3d&& other) noexcept;