Make Kruskal spanning tree accept weight vector instead of matrix

[matbesancon]

Kruskal seems to be the only one in the current implementation that accepts a matrix and converts it to a vector.
Instead of this additional allocation, I refactored the two steps to be able to call the function with a weight vector directly, it was the form I needed in the first place for something in which I manipulate weight vectors

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.19%. Comparing base (a716a84) to head (7ee34e9).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #486   +/-   ##
=======================================
  Coverage   97.19%   97.19%           
=======================================
  Files         121      121           
  Lines        7105     7107    +2     
=======================================
+ Hits         6906     6908    +2     
  Misses        199      199           

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