Modified default tuning parameters. Added APLRTuner

Modified default hyperparameters based on empirical tests on several openml and pmlb datasets. Learning rate (v) was increased to 0.5 from 0.1, min_observations_in_split was decreased to 4 from 20, ineligible_boosting_steps_added was increased to 15 from 10 and max_eligible_terms was increased to 7 from 5.

Also added the APLRTuner object which simplifies tuning of APLR. Se the example folder.