Scikit-learn API enhancements

Below are some ideas for future enhancements for `pennaipy`:

- refine default values of [parameters in PennAI](https://github.com/EpistasisLab/pennai/blob/v0.17/ai/sklearn/pennai_sklearn.py#L48-L71) for better AutoML application
- support more classifiers and regressors in the [PennAI configurations](https://github.com/EpistasisLab/pennai/tree/master/ai/sklearn/config)
- update knowledgebase every half year or quarter based on latest version of scikit-learn/xgboost
- Add support of cuML for big data analysis with GPU (similar with [TPOT cuML](https://epistasislab.github.io/tpot/installing/#installation-for-using-tpot-cuml-configuration))