message: Add Planet::position() read method
diff: @@ -35,6 +35,7 @@ public:
  PlanetType type() const { return mType; }
  + NAS2D::Point<int> position() const { return mPosition; }
  void position(const NAS2D::Point<int>& point) { mPosition = point; }
  void position(int x, int y) { mPosition = {x, y}; }
  