Preparation of CRAN release

[mayer79]

This PR prepares CRAN release 0.4.0

[codecov-commenter]

Codecov Report

Merging #110 (aafa414) into main (a690ff6) will increase coverage by 1.62%.
The diff coverage is 100.00%.

❗ Current head aafa414 differs from pull request most recent head 4b25bb0. Consider uploading reports for the commit 4b25bb0 to get more accurate results

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@@            Coverage Diff             @@
##             main     #110      +/-   ##
==========================================
+ Coverage   94.91%   96.54%   +1.62%     
==========================================
  Files           6        6              
  Lines         433      434       +1     
==========================================
+ Hits          411      419       +8     
+ Misses         22       15       -7     

Files	Coverage Δ
R/kernelshap.R	98.93% <ø> (ø)
R/methods.R	90.32% <100.00%> (+23.65%)	⬆️
R/permshap.R	100.00% <ø> (ø)