Add a velocity to a position
nat_pos_plus_vel_cpp(cl, vl, vl_neg, n_arcs)
the new position by reference and the new number of arcs by return
the position's causal list
the velocity's positive causal list
velocity's negative causal list
number of arcs present in the position. Remainder: can't return integers by reference, they get casted to 1 sized vectors