Model SignalArrow defines an arrow that is
dynamically visualized at the location where its frame_a is
attached. The position vector from the tail to the head of the
arrow, resolved in frame_a, is defined via the signal vector of the
connector r_head (Real r_head[3]):
The tail of the arrow is defined with parameter r_tail with respect to frame_a (vector from the origin of frame_a to the arrow tail).