model.desc: A description of the model including the
three integer components (p, d, q) are the AR order,
the degree of differencing, and the MA order.
sigma: The square root of the estimated residual variance
logLik: The data's log-likelihood under the model
AIC: The Akaike Information Criterion
BIC: The Bayesian Information Criterion
ME: Mean error
RMSE: Root mean squared error
MAE: Mean absolute error
MPE: Mean percentage error
MAPE: Mean absolute percentage error
MASE: Mean absolute scaled error
ACF1: Autocorrelation of errors at lag 1