[Method] Add XGBoost

[sarthakpati]

https://xgboost.readthedocs.io/en/latest/python/python_intro.html

Ref:

• https://www.frontiersin.org/articles/10.3389/fnins.2019.01449/full
• https://github.com/s-shailja/BraTS2018-tumor-segmentation/blob/main/ensemble.py