add edge_probability in tutorial.grid
add paths in test/*.py automatically
mv c++ tests to src/test
mv test to graphillion/test, and edit setup.py
remove setset(set) constructor
small modifications
a graph can be represented by [edges+] without using set
fix __repr__
check argument of set_universe()
small modifications
add choose in zdd.cc
add lexicographical iterator and make it as default
skip removing selected sets in randomize(), and use default iter in python

- skip removing selected sets in randomize() if large setset
- use defaul iterator in python as well as c++
remove is_bot(f) and is_top(f) and use f == bot() or top()