change flip() as non-const method in c++ and py, and do some more
- flip(...) modifies this/self and is not a const method
- update documents in graphset.py a little
replace include() and excplide()
- in setset, replace include() with supersets(), and replace exclude() with
  non_supersets()
- in GraphSet, replace include() with including(), and replace exclude() with
  excluding()
move including() and excluding() from lookups to binary_operators
replace subgraphs() with included() in GraphSet
rename same_size() and same_len()
- in c++, rename same_size(n) to size(n)
- in python, rename same_len(n) to len(n)
add choice() to python
rename get_universe() to universe() in python
rename some methods for frontier-search
- in c++, rename FrontierSearch() to SearchGraphs()
- in python, rename subgraph() to graphs()
add an argument to GraphSet.graphs() and do some more
- add 'search_space' argument to GraphSet.graphs()
- accept int as well as range as 'degree_constraints' and 'num_edges' arguments
dynamically add gs.graphs() as an instance method in GraphSet.__init__()