Subtracts two natPositions to obtain the natVelocity that transforms ps1 into ps2
nat_pos_minus_pos_cpp(ps1, ps2, vl, vl_neg)
the velocity's causal lists by reference and the number of operations by return
the first position's causal list
the second position's causal list
the natVelocity's positive causal list
the natVelocity's negative causal list