Feat/add pre trained model xgbse 1st step

[davivieirab]

• Add the possibility of using a pre-trained XGB estimator for the 1st step model in both XGBSEDebiasedBCE and XGBSEStackedWeibull

Add tests:

• function to assert a valid xgb estimator is used as pre-trained model
• model with and without the pre-trained xgb is fitted properly for both modules

Add docs:

• example on how to use a pre-trained xgb model for both modules