Infinite Moving average representation
[mainf_proxy]=mainf(ar,ma,nbterms)
* ar = the ar part = a (p x 1) or (1 x p) vector
* ma = the ma part = a (q x 1) or (1 x q) vector
* nbterms = # of terms developped (optional, default = 100)
* mainf_proxy = the nbterms first terms of the Infinite Moving average representation