* add back train method but mark as deprecated * fix scalastyle error * fix the persistence of XGBoostEstimator * test persistence of a complete pipeline * fix compilation issue * do not allow persist custom_eval and custom_obj * fix the failed tesl
* add back train method but mark as deprecated * fix scalastyle error * fix the persistence of XGBoostEstimator * test persistence of a complete pipeline * fix compilation issue * do not allow persist custom_eval and custom_obj * fix the failed tesl