Redesigned, restructured and accelerated

1. Increased performance of models (accelerated by 15-40 percent). Load profiling was performed via profile and snakeviz.
2. Restructured external models from stratified_model.py to a separate module.
3. Realized wrappers of classical AFT and CoxPH models.
4. Build a parallelized Bootstrap model
5. Refactor scipy code of sf and isf counting (statistics values and p-values).
6. Added documentation