[Camera] fix camera frame transformation + relative in SemanticCamera

The frame of the camera, available using self.position_3d
is now computed using a frame transformation to be consistent
with MORSE conventions.

Also change the builder part, so that when doing:
robot = ATRV()
camera = VideoCamera()
robot.append(camera)

The camera is oriented the same way as the robot (regaring objets
themselves and their position_3d).
And robot.rotate and camera.rotate behave the same way.

SemanticCamera now uses transformations to compute the position
of detected objects in the camera frame.