Slight optimization

[mayer79]

• Replacing repeated rowpaste() by <-substr()
• Speed-up for single-output predictions

[codecov-commenter]

Codecov Report

Merging #115 (6797de7) into main (def8fe5) will decrease coverage by 0.35%.
The diff coverage is 91.30%.

❗ Current head 6797de7 differs from pull request most recent head 191b310. Consider uploading reports for the commit 191b310 to get more accurate results

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@@            Coverage Diff             @@
##             main     #115      +/-   ##
==========================================
- Coverage   96.59%   96.24%   -0.35%     
==========================================
  Files           6        6              
  Lines         440      453      +13     
==========================================
+ Hits          425      436      +11     
- Misses         15       17       +2     

Files	Coverage Δ
R/utils.R	97.39% <100.00%> (+0.24%)	⬆️
R/utils_permshap.R	100.00% <100.00%> (ø)
R/permshap.R	96.72% <50.00%> (-3.28%)	⬇️