Adding code coverage workflow

[gvegayon]

• Adds coverage report via pytest-cov.
• Sets up codecov.io and uploads report.
• Adds a badge to README.md
• Renames existing workflows to have shorter badge names.

[codecov-commenter]

Welcome to Codecov 🎉

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 ☂️

[dylanhmorris]

I somewhat preferred the more verbose test_model.yaml and test_pipeline.yaml names for the workflow files, but I am open to this change, and I won't block this on that account alone. Everything else looks good.

[gvegayon]

I somewhat preferred the more verbose test_model.yaml and test_pipeline.yaml names for the workflow files, but I am open to this change, and I won't block this on that account alone. Everything else looks good.

No strong opinion on that, so I'll revert it before merge.