transformation of VARMA model into its reduced form
[F,A,V,G]=theta2arma(theta,theta2mat)
theta = (npx1) vector of parameters
theta2mat = a string vector of instructions that transforms
F = the MA part of the model
A = the MA part of the model
V = the variance of residuals
G = the exogenous part of the model
[F,A,V,G]=theta2arma(theta,theta2mat) // Taken from function theta2fr. Since this function is an intermediate calculation in an VARMA estimation, it would be diffcult // to use it outside this framework.