hill_cipher.py: gcd() -> greatest_common_divisor() (#1997)

* hill_cipher.py: gcd() -> greatest_common_divisor()

* fixup! Format Python code with psf/black push

* import string

* updating DIRECTORY.md

* Change matrix to array

Add more tests

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
Co-authored-by: John Law <johnlaw.po@gmail.com>