allow survival predictions for VI

[bcjaeger]

Something I noticed while updating aorsf-bench in response to https://github.com/bcjaeger/aorsf-bench/pull/7 (👋 @darentsai):

in the paper associated with aorsf-bench, all benchmarks of variable importance were based on predicted survival probability. Since then, aorsf was updated to automatically use mortality predictions for VI computation because it was much simpler and didn't require specification of a prediction horizon. However, I think it would be ideal to retain the methods that were used in the paper, so I think it would be helpful to add an option to make variable importance for survival forests use predicted survival probability.