Feat/countdown scoring

[joesharratt1229]

The following pr adds an implementation of the score_answer method for the countdown game.

Files amended

• games/countdown.py - adds score_answer method, which checks if user provided answer matches the target. Handles cases of incorrectly formatted strings, null answers and partial rewards for partial solutions
• tests/test_countdown.py - checks within test_coundown_games_items the reward matches expected outputs.

[andreaskoepf]

Nice, please run pre-commit run -a for standard black formatting. For future you can run pre-commit install in the repo dir to install a local hook which runs the pre-commit checks while committing locally.