Pass which.curve to PredictionSurv

[bblodfon]

Description

We cannot currently pass and store the which.curve parameter from a learner like BART into the PredictionSurv object. Currently, which.curve is just used to calculate the crank in .surv_return(). Therefore, all survival metrics or when combining predictions (via c.PredictionDataSurv) use the median curve (0.5).

TODOs

1. pass the which.curve in PredictionSurv object somehow
2. whenever we need to reduce the survival array to a matrix, an Arrdist via the p$distr field to a Matdist or convert with as.Distribution(...), we will extract the which.curve matrix, not the median
3. combining a mix of survival arrays with matrices is now done by reducing the arrays to the median survival matrices but it should be the which.curve ones