use tempfile in test.py
divide universe() into set/get, and edit setset doc for GraphSet
update doc for setset
put all method descriptions into graphset.py
small modifications
invert -> flip, complement -> invert
add complement graphs and flip()
small modifications
apply composite design pattern to GraphSet
small modifications
small modifications
small modifications
comment out quotient, remainder, join, and meet
small modifications
small modifications
refactoring on tests
divide test.py
add tests to GraphSet