[XGBoost] - New parameter CPU DVFS

[ArneTR]

If the OS is configured with DVFS the power curve is strongly non linear in relation to load.

This should be a parameter in the data that we train the model with.

After making this availabe in the source data we would also need a flag to feed into the model as possible input parameter.

This parameter is very important because some machines, like the Github VMs, run only with fixed CPU frequency.