Collect coverage of numba decorated code

[dstansby]

Fixes #110. This adds a new CI workflow that only runs on main that disables numba, and therefore collects code coverage for the parts of the code that would otherwise be optimised by numba. The tests are slow (~20 mins), but not unreasonably slow, so running them only on commits to main seems like a good compromise.

I've also enabled carryforward flags for these parts of the code in codecov. This will automatically carry forward the covered lines on main to pull requests, so coverage isn't going up and down in main compared to pull requests where the new workflow isn't run.

[github-advanced-security]

You have successfully added a new SonarCloud configuration ``. As part of the setup process, we have scanned this repository and found no existing alerts. In the future, you will see all code scanning alerts on the repository Security tab.

[dstansby]

This seems to work well, but the tests with Numba turned off takes 13 minutes to run. I'm on the fence about whether this is too slow or not for regular CI. I will take a look and see if there's a way to run the slower Numba off run only on merges to main, and have codecov play nicely and not complain that every PR decreases coverage.

[dstansby]

Currently wainting on neuroinformatics-unit/actions#18