Commons module refactoring & documentation

[marfox]

• [x] write exhaustive docstrings for CLI commands;
• [x] add help kwargs in click decorations;
• [ ] write signatures & docstrings for public functions/class methods;
• [ ] fix sphinx warnings: cd docs && pipenv run make html;
• [ ] fix pylint errors: pipenv run pylint -j 0 -E MODULE;
• [ ] fix pylint warnings: pipenv run pylint -j 0 -d all -e W MODULE:
• [ ] reduce complexity: pipenv run flake8 --select C90 --max-complexity 10 MODULE;
• [ ] apply relevant pylint refactoring suggestions: pipenv run pylint -j 0 -d all -e R MODULE;
• [ ] resolve TODOs.

[marfox]

Up to now, the issue-291 branch contains a few docstrings of public functions and some refactoring that has potentially gone wrong.