xgboost updates

[thesps]

Initiated by #46

• fix the xgboost converter when the updater is not the default
• fix xgboost multiclass conversion
• accept XGBClassifier and XGBRegressor objects for conversion as well as Booster as before
• simplify treeToDict for xgboost converter by using trees_to_dataframe method of Booster
• add xgboost converter test
• generally allow converters to import non-required modules and try to import them in the __init__.py

Not relate to xgboost:

• fix to C++ backend for importing multiple pybind11 bridge modules (e.g. different models)