narsimul armax simulation (using rtitr)

## Syntax

                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)

## Description

ARMAX simulation. Same as arsimul but the method is different the simulation is made with rtitr

## Examples

    a=[1,-2.851,2.717,-0.865].*.eye(2,2)
    b=[0,1,1,1].*.[1;1];
    d=[1,0.7,0.2].*.eye(2,2);
    sig=eye(2,2);
    ar=armac(a,b,d,2,1,sig)
    u=rand(1,10,'normal');
    y=narsimul(ar,u)