feat: adding code cov for PyTest including statement & branch coverage

[abanuelo]

Adding pytest-cov, a plugin on top of pytest for code coverage along with coverage tool in pyproject.toml:

• shows branches in test files
• show percentage of statements and branches covered
• lines of code missed or needing of testing
• sets a minimum average code coverage threshold of 80% for all test files
• Create new GitHub Workflow for coverage

[Eric-Vin]

Looks great to me! Just have two questions:

• Should we run the example tests as well when calculating coverage? I'm thinking probably not, since the test suite itself should ideally cover all behavior, but I know there's some behavior in certain interfaces not tested in the test suite but covered by the examples.
• Will this calculate coverage for scenic interface files? I know some of them have quite complicated behavior, such as the ground surface generation for Webots.

[abanuelo]

1. Eventually, yes! examples/ was originally excluded from the CI and this PR does not change that. I think we can consider re-adding it to both CI and coverage when they stabilize.
2. No. Everything within the src/scenic/simulators/* has been excluded from coverage.

[Eric-Vin]

Whoops, sorry! I meant to say Scenic model files, not the interfaces. Those are .scenic files though so I'm not sure if we could even use the same infrastructure to measure coverage for those.

[abanuelo]

Correct, I don't think we could use the same code coverage frame work since technically it looks for code coverage around .py files. I can look into potentially extending the existing coverage library to find line/branch coverage for .scenic files but that would require some work.

The question here is, how beneficial is line coverage for overall scenic simulation files? My first impression is we just care about expected inputs and outputs of those simulations to be correct and less about line coverage.

[dfremont]

The Scenic world models that Eric is talking about are general libraries used across many scenarios, so we should test their functionality and line coverage would be useful. But let's leave that for later -- it would require writing a plugin for coverage.py (the underlying library being used here) to give it support for Scenic files.

[codecov[bot]]

Welcome to Codecov :tada:

Once you merge this PR into your default branch, you're all set! Codecov will compare coverage reports and display results in all future pull requests.

Thanks for integrating Codecov - We've got you covered :open_umbrella: