XGBoost 2.0.0

[thesps]

The XGBoost converter is not working for XGBoost 2.0.0. This issue is for tracking the development of the fix.

[thesps]

Brought to my attention by @brightneedle in #41

[thesps]

This xgb_2 branch has the first pass of fixing conversion for XGBoost >= 2.0.0. It parses models correctly but the numerical output of the converted conifer model mismatches the original xgboost model worse than XGBoost 1.X.Y models. This can be seen in the xgboost converter test.