Add code coverage

[ginic]

Code Coverage: We could add a tool like Coverage.py to measure code coverage. I have also used pytest-cov in a project. Although maximizing coverage is a flawed metric to optimize for, it's still helpful and improves test writing.

Definition of done:

• Set up pytest-cov to be run to determine test coverage for a repo
• Test coverage is automatically determined and displayed on the GitHub repo for a project (using GitHub actions)
• Update README testing section to describe what test coverage is, its limitations and how to determine test coverage