Add interface for multi-output models

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[codecov-commenter]

Codecov Report

Merging #33 (ae9dfa7) into master (cec10f9) will decrease coverage by 1.47%.
The diff coverage is 13.63%.

❗ Current head ae9dfa7 differs from pull request most recent head 7f61bd1. Consider uploading reports for the commit 7f61bd1 to get more accurate results

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@@            Coverage Diff             @@
##           master      #33      +/-   ##
==========================================
- Coverage   68.91%   67.45%   -1.47%     
==========================================
  Files          19       19              
  Lines         830      851      +21     
==========================================
+ Hits          572      574       +2     
- Misses        258      277      +19     

Files	Coverage Δ
R/unify.R	88.88% <ø> (ø)
R/unify_ranger_surv.R	92.59% <100.00%> (+0.13%)	⬆️
R/model_unified.R	69.23% <0.00%> (-20.77%)	⬇️
R/treeshap.R	70.12% <13.33%> (-14.00%)	⬇️