Include XGBoost Model

[KaiyiLiu1234]

As requested by @rootfs, XGBoost model should be included in the pipeline as another non LR model for inference.

[ArneTR]

Hey Kepler team,

we already have developed an XGBoost model that runs via the based XGBRegressor based on the SPECPower data.

I figure this could be quite a fit for Kepler.

• https://github.com/green-coding-berlin/spec-power-model/blob/main/README.md

Since we are continously working on the model in terms of improvements we have it factored out as a separate repository from our other tools.

Is it interesting for the Kepler project to integrate models via git submodules?