autocovariance generating function
ac = agf(w,AR,MA)
* w = point where to evaluate
* AR = matrix of AR coefficients AR = [A1 .. Ap]
* MA = matrix of MA coefficients MA = [B1 .. Bq]
* ac = AGF for VARMA
ac= agf(w,[0.1 0.2],[0.43 0.21])