Add pytest-cov to test coverage of the generated Python classes

It would be great if we could use [pytest-cov](https://pytest-cov.readthedocs.io/en/latest/) to test how much of the generated Python Data Classes in crdch_model we're currently testing in the Example Data Workflow. Sujay suggested it at https://github.com/cancerDHC/example-data/pull/41#discussion_r755635559, but I think we should implement and test this in a different PR.