Support for censored probabilistic regression (survival analysis; AFT models)

[gmgeorg]

Thanks for this great library!

I was wondering if you plan to include censored loss functions as well to allow for censored probabilistic regression. Similar to what XGB AFT models do, but with a non-constant scale parameters in the distribution models (https://xgboost.readthedocs.io/en/stable/tutorials/aft_survival_analysis.html).

[StatMixedML]

@gmgeorg Thanks for your interest in the project. Great suggestion, I`ll add it to my ToDo.

[John-Curcio]

This PR attempts to check that off the list