Added pytest coverage report. Part of #31.

[julianharty]

Added the pytest coverage module and used it to generate our first coverage report.

For general info on this module:

• https://pytest-cov.readthedocs.io/en/latest/readme.html

To get the coverage report working

• https://stackoverflow.com/questions/72986258/pytest-cov-showing-no-coverage

And to generate the badge using genbadge

• https://smarie.github.io/python-genbadge/

The coverage.xml report is created in reports/coverage/ but excluded in .gitignore

We can run the tests and/or coverage by hand for now; let's integrate it so it's run automatically in the near future.