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 min 3 3

 n 3 s

 n 2 t

 a 3 1 20 20

 a 1 2 20 10

 a 1 2 20 0