Increased interpretability

Increased interpretability in APLRRegressor:

• Added methods calculate_feature_importance and calculate_term_importance. These estimate feature and term importance respectively on new data.
• Added method get_term_importance which returns estimated term importance in the training data for each term in the model.
• Terms in the model are now sorted by estimated term importance in the training data.
• Slightly changed feature importance calculation methodology.
• Renamed method calculate_local_feature_contribution_for_terms to calculate_local_term_contribution.