Set up a code coverage workflow job

[ddundo]

At some point I looked into setting up a code coverage check that would fail if the coverage decreased due to a PR but I couldn't find a decent solution for free. There's a tool called code-cov but I think you have to pay for it.

Originally posted by @jwallwork23 in mesh-adaptation/movement#139 (comment)

I'm perhaps overly confident, but I could probably code this up in two steps:

• The test suite on the main branch uploads the coverage report as an artifact (with maximum allowed retention time)
• The test suite on a PR compares its new coverage report with the one uploaded from the main branch

[ddundo]

I also wouldn't be surprised if there's already an action for this. I'll check

[ddundo]

Codecov seems to be free and unlimited for open-source public repos: https://about.codecov.io/pricing/

And there is indeed an action already: https://github.com/marketplace/actions/code-coverage-report-difference https://github.com/GreatWizard/coverage-diff-action

[jwallwork23]

And there is indeed an action already: https://github.com/marketplace/actions/code-coverage-report-difference

We agreed to go for this one.