GC-45: Update copyright information:

- Changes to code base were previously done in 2020 as well (based on
git history).

GC-11: Added a help subcommand to the tool:

- Updated functional test for checking on the help subcommand as well.
- Implemented the 'help' subcommand.
- Updated required function signature for the subcommand_parser
decorator (functions should return ArgumentParser instance).
- Fixed a typo in inline documentation.
- Updated the test for subcommand_parser decorator to be a more robust
when checking if function registration works.

GC-11: Implemented decorator mechanism for registering subcommands:

- Added decorator that will register the provided functions.
- Added a simple interface function for fetching list of such
registered functions.