Prevented re-calculating already calculated objectives.
Added regime indicator array to the return value of build_exog
Added a special cast for selecting SETAR(1) hyperparameters.
Added mechanism for getting alternative models in SETARResults, especially the SETAR(1) aka AR case
Added new SETARResults properties, refactored some to use new alternate model caching.
Refactored predict and forecast to take delay and thresholds as required parameters to make them more general / flexible.
Added test between two SETAR orders, for the homoskedastic, regime heteroskedastic, and general form heteroskedastic cases.
Minor pep8 corrections.
Added check for invalid heteroskedasticity type in order_test