Document that --cov is supported

[fangohr]

Example at https://twitter.com/ProfCompMod/status/1055845233295060992.

Attached file is notebook from that tweet (with wrong extension as GitHub didn't allow upload for ipynb file).
nbval-coverage copy.ipynb.txt

[vidartf]

This is currently only mentioned in the README (https://github.com/computationalmodelling/nbval#coverage), but should also be mentioned in the docs. The attached notebook should be enough for now.

[fangohr]

Nice.

Can we move the notebook into the documentation?

[banesullivan]

Is --cov-append supported?

I have a package that has most of its testing in standard .py files but we've been adding IPython features that need to be executed in a notebook. Ideally, I'd like to append the coverage report.

[takluyver]

Try it and let us know!

[jetheurer]

--cov-append did not work for me

[vidartf]

I'm not overly familiar with --cov-append. Can someone give some more details on use cases? Which types of log outputs are you trying to combine?

[hameerabbasi]

--cov stalls the whole notebook tests for me currently. They timeout and then error.

[vidartf]

@hameerabbasi Thanks for your feedback! Would you mind opening a new issue on this with some more details on what causes this problem (steps to reproduce)?

[hameerabbasi]

Done in #116! Let me know if you need anything else to help diagnose the issue.