fix repr
add maximize/minimize from optimize
merge in/exclude edge and vertex
do refactoring in setset.py and graphset.py
add graft() and prune()
add invert(e), insert(e), and erase(e) in c++
small modifications
remove graft(e) and prune(e); add invert(e), add(e), and remove(e) in python
rename to GraphSet from graphset
replace nullptr with NULL
remove method hooks; increase RecurLimit; add interation in large-scale tests
de-hashable