This action class can snap (set) the relative-zero marker in the middle between two points chosen by the user.
The middle point, by default, rests at a poistion 50% relative to the position of the first point chosen, that is, at the center of the imaginary line connecting the two user-defined points.
This percentage, however, can be modified by the user in order to place the marker at a different position along the imaginary line.
