Add xgboost model

[KatrionaGoldmann]

Summary

Adds the xgboost model fitting and predictions

What should a reviewer concentrate their feedback on?

• [ ] Updating requirements file
• [ ] Everything looks ok?

Acknowledging contributors

• [x] All contributors to this pull request are already named in the table of contributors in the README file.
• [ ] The following people should be added to the table of contributors in the README file