Add a velocity to a position
nat_pos_plus_vel_cpp(cl, vl, vl_neg, n_arcs)
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
the new position by reference and the new number of arcs by return