merge num_comps argument into vertex_groups in graphs() in GraphSet
add syntax-sugar methods for frontier-search in graphset.py
- add connected_components(), cliques(), trees(), forests(), cycles(),
  and paths()
- fix a bug on num_comps in GraphSet.graphs()
fix a bug on normal iterator in c++
add tests for the previously-fixed iterator bug
update document of rand_iter on the random number generator
add large-scale tests for frontier-search