Infinite Moving average representation
[mainf_proxy]=mainf(ar,ma,nbterms)
ar= the ar part = a (px1) or (1xp) vector
ma = the ma part = a (qx1) or (1xq) vector
nbterms = # of terms developped (optional, default = 100)
mainf_proxy = the nbterms first terms of the Infinite Moving average representation