Improved InvalidRegimeError parent class
BUG Fixed a problem caused b/c self.exog gets created (i.e. overwritten) during fit()
BUG Raised appropriate exception if hyperparameter search returns no results (e.g. threshold grid size too small)
ENH: Allowed passing additional constructor parameters to _get_models
ENH: Updated bootstrap in order_test() to use joblib for parallel execution when possible.
Restricted order testing to only selecting the thresholds and not the delay parameter in bootstrap iterations. See accompanying code to Hansen (1999)