Added ranger_surv.unify for random survival forests with {ranger}

[kapsner]

This PR introduces the function ranger_surv.unify in order to compute tree-shap values for random survival forest models built with the {ranger} 📦.

• [x] roxygen documentation (including example)
• [x] unit tests
• [x] dependencies to DESCRIPTION

(Partly) addresses #19

Edit: The predictions for the ranger-survival models are computed using the sum over chf as explained here.

[pbiecek]

thanks!