Add Test Coverage Report on the CI/CD and README

[danlessa]

We could use pytest-cov and codecov.io to produce dynamic stats on the test coverage over the repo. This could be potentially helpful for prioritizing maintenance work.

For instance, the current coverage is at 53%. Here's the output from pytest --cov=cadCAD --cov-report html

[linear]

CORE-102 Add Coverage Reports on the CI/CD