Support for other survival tree-based models

[krzyzinskim]

Adding support for survival models other than ranger (see #24). The source code for the ranger can be followed. Functionality for randomForestSRC should be relatively easy to implement.

[mayer79]

@krzyzinskim If I recall correctly, {randomForestSRC} even supports multi-output regressions (which is nice).

[krzyzinskim]

Hmm, in that case, I'll try to rethink the structure for handling non-single-number predictions so that multi-output regression can be also supported.

[mayer79]

(Dito for XGBoost 2.0). The output structure can be like classification, I'd say: a list of matrices/arrays (?)