Wrapper for `shap.TreeExplainer` for SurvSHAP

[krzyzinskim]

• implement shap.TreeExplainer for local explanations (PredictSurvSHAP)
• implement shap.TreeExplainer for global explanations (ModelSurvSHAP)
• check the other explainer types from shap library

[krzyzinskim]

implementation can be based on #15