Set up codecov workflow and badges

[tsmbland]

Description

Adds a step to the CI workflow to upload the coverage report to codecov, and link this page from the documentation.

NOTE: the workflow is currently failing, but we believe it will only work once it has been merged to main

This replaces a half-implemented workflow using the mkdocs-coverage package which would be considerably more effort to set up properly. I also needed to change the coverage report from html to xml to get it working with codecov.

Also adds badges for codecov and PyPI to the readme

Fixes #287

Type of change

• [ ] Documentation (non-breaking change that adds or improves the documentation)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Optimization (non-breaking, back-end change that speeds up the code)
• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] Breaking change (whatever its nature)

Key checklist

• [ ] All tests pass (eg. python -m pytest)
• [ ] The documentation builds and looks OK (eg. python -m sphinx -b html docs docs/build)
• [ ] Pre-commit hooks run successfully (eg. pre-commit run --all-files)

Further checks

• [ ] Code is commented, particularly in hard-to-understand areas
• [ ] Tests added or an issue has been opened to tackle that in the future. (Indicate issue here: # (issue))

[tsmbland]

@dalonsoa Please could you generate a codecov token and add to the repository secrets?

[dalonsoa]

Done. It should be available as secrets.CODECOV_TOKEN, I think.

[tsmbland]

Thanks, although it's not finding it for some reason...

[tsmbland]

@dalonsoa You'l have to merge it for me as I'm blocked by branch-protection rules

[tsmbland]

Still not working 😭 😭 😭 😭