This utility function is useful to use in the portmanteau functions,
BoxPierce, MahdiMcLeod, Hosking,
LiMcLeod, LjungBox, and portest.
GetResiduals() function takes a fitted time-series object
with class "ar", "arima0", "Arima",
("ARIMA forecast ARIMA Arima"), "lm", ("glm" "lm"),
"varest", or "list".
and returns the residuals and the order from the fitted object.