Model SignalArrow defines an arrow that is dynamically visualized
at the location where its frame_a is attached. The
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 quantity parameter defines what the vector represents, allowing
tools to scale e.g. forces and torques differently in a consistent way.
For the default value RelativePosition the obvious scaling is
1 and the relative position is shown as is.
The tail of the arrow is defined with input r_tail
with respect to frame_a (vector from the origin of frame_a to the arrow tail).