Updated post_pred_dist() method to allow for different set of predictors

[devindg]

• posterior_predictive_distribution() has been renamed to post_pred_dist() for brevity
• post_pred_dict() now accepts 'predictors' as argument. This is to allow the user to test sensitivity of the model's in-sample predictions to new predictor data.
• Docstrings updated/corrected to reflect previous changes to priors
• _ar_state_post_upd() method has been moved inside BayesianUnobservedComponents as a static method
• _simulate_posterior_predictive_response() has been removed as it's no longer needed. post_pred_dist() takes its place.