combineMA function
Implementation
List<double> combineMA(List<double> theta, List<double> Theta, int s){
final non=<double>[1.0, ...theta]; final seas=_seasonalPoly(Theta, s, ar:false);
final c=_polyMul(non,seas); return [for (int k=1;k<c.length;k++) c[k]];
}