Implement XGBoost classification and regression algorithms

Feature description

machine_learning/xgboost_classifier.py and machine_learning/xgboost_regressor.py are how-tos since they both just use an existing library for the actual ML algorithms.

My understanding is that #7106 and #7107 were merged (not without difficulty) and the author was warned not to contribute such how-tos in the future. However, I think these algorithms should still be implemented at some point if the files are to remain in the repo, so I thought I should open an issue to bring some attention to it.

If anyone wants to implement either of these two algorithms (as in not relying on an existing library for the bulk of the algorithm), feel free to just open a PR—no need to request an assignment.