A numerical vector giving the values \(p\), \(d\) and \(q\) of
the ARIMA or ARFIMA model. For a seasonal ARIMA model, the returned vector
contains the values \(p\), \(d\), \(q\), \(P\), \(D\), \(Q\) and
\(m\), where \(m\) is the period of seasonality.