Support for survival functions

[ipa]

This is related to: bambinos/bambi#541

To fit a survival function with censored data a response consisting of two components is required. One component depicting the time-to-event and one component depicting the censorship/event status.

Proposing similar to the Surv function in R (https://www.rdocumentation.org/packages/survival/versions/3.3-1/topics/Surv).

Surv(time, event)
event: 0 no event / right censored, 1 event

Propose to implement this as transformation. The eval() method would then simply return a 2xn matrix with time in the first column and event status in the second.

[tomicapretto]

I'm closing this since bambinos/bambi#581 as well. Thanks again!