Update + include new learners

[bblodfon]

• [ ] BART
  • Include it in SurvLPS
  • Tuning: theta, omega, a, b, rho, k, power, base, tau.num, ntree, ndpost, nskip
  • ntree => 50 - 200 and k matters (suggestion by BART author)
• [ ] aorsf
  • test in eFS, update parameters and importance
  • Check issue
• [ ] CoxNet
  • distr prediction by default (no pipeline needed)
• [ ] xgboost
  • distr prediction in cox objective by default (update todo)
  • Simplify the search space