XGBoost notebook overwrites same data locations

[athewsey]

The template notebook for training & evaluating XGBoost overwrites the same local & S3 locations each time the train/val/test code is run.

Users should be re-running this code each time they try a new Data Wrangler flow, so it should be refactored to save to a new location each time so the history is still available (in case an earlier experiment proved to be better!)