Re-commit partial completion of #42: Refactor docstrings according to PEP 257 using "pep257":
* session.py: global_session
* filters.py: pop_self
filters.py: fill_empty
helpers.py: create_app
helpers.py: delete_app
helpers.py: get_app_extras
helpers.py: install_app
helpers.py: pre_made_app_filter