bp_gap.model: Time series model for the best-practice gap
In MortalityGaps: The Double-Gap Life Expectancy Forecasting Model

Time series model for the best-practice gap

1	bp_gap.model(X, benchmark, arima.order, drift)

`X`	Input data object generated by prepare_data function.
`benchmark`	Fitted values given by BP linear model.
`arima.order`	A specification of the the ARIMA model to be used in fitting the best-practice gap. The ARIMA order is country specific. The three integer components (p, d, q) are the AR order, the degree of differencing, and the MA order. Format: numerical vector of length 3. If `arima.order = NULL` the function conducts a search over possible models according to AIC. See `auto.arima` for details.
`drift`	Indicate whether the ARIMA model should include a linear drift term or not. Type: logical value. If `drift = NULL`, it will be estimate automatically.