fix missing serde derives on KMeansInit

[pYtoner]

I changed my linfa_clustering version from 0.3 to 0.4 and I was unable to use serde with linfa_clustering::KMeans. I looked at the errors and KMeansInit didn't have any serde derives / implementations.

[bytesnake]

can you run rustfmt, then I will merge

[codecov-commenter]

Codecov Report

Merging #128 (fe14809) into master (48c9bd7) will decrease coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #128      +/-   ##
==========================================
- Coverage   59.93%   59.87%   -0.06%     
==========================================
  Files          74       74              
  Lines        7006     7006              
==========================================
- Hits         4199     4195       -4     
- Misses       2807     2811       +4     

Impacted Files	Coverage Δ
algorithms/linfa-clustering/src/k_means/init.rs	87.23% <ø> (ø)
...rithms/linfa-trees/src/decision_trees/algorithm.rs	51.08% <0.00%> (-1.45%)	⬇️

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[pYtoner]

Done. Sorry forgot to run fmt because it was such a small change 😃

[bytesnake]

Done. Sorry forgot to run fmt because it was such a small change smiley

no worries, for that we have the CI 😄