Represents time series linear regression models with "trend" and "season" variables.
buildModel() passes on fitting and forecasting functions to FitModel's method setFittedFcasted; applies Box-Cox transformation and bias-adjustment; passes function which checks whether residuals are not independently distributed
useModel(fcast_period) calls FitModel's method executeUseModel to fit model and forecast fcast_period-step ahead future; passes on the same fitting and forecasting functions as buildmodel()