workaround for sort ambiguity in cat_features

[CarloLucibello]

Fix #104, fix #98

[codecov]

Codecov Report

Merging #105 (aea736a) into master (364c57e) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #105      +/-   ##
==========================================
+ Coverage   83.91%   83.93%   +0.01%     
==========================================
  Files          14       14              
  Lines        1144     1145       +1     
==========================================
+ Hits          960      961       +1     
  Misses        184      184              

Impacted Files	Coverage Δ
src/GNNGraphs/utils.jl	70.11% <100.00%> (+0.34%)	⬆️

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