for backwards compatibility, made collapse operator arguments in new liouvillian function take an empty list by default
pep8 fixes, code cleanup, docstring updates
Merge pull request #180 from jrjohansson/fix-liouvillian-args

Fix liouvillian args
pep8 fixes
pep8 fixes
pep8 fixes
pep fixes and code cleanup
more pep8 fixes
pep8 fixes for unit tests
Merge pull request #181 from jrjohansson/code-cleanup-pep8-fixes

Code cleanup pep8 fixes
merge master