Kruskal alg. (src/hugo/kruskal.h, src/test/kruskal_test.cc) is (almost) done.
- Some input adaptor is still missing.
- The class and function names should be revised.
- Docs still needs some improvement.

p mat 5 5

a 1 3

a 2 3

a 3 5

a 4 3

a 5 4