armax simulation (using rtitr)
z = narsimul(a, b, d, sig, u) z = narsimul(a, b, d, sig, u, up, yp, ep) z = narsimul(ar, u) z = narsimul(ar, u, up, yp, ep)
ARMAX simulation. Same as arsimul but the method is different the simulation is made with rtitr