Add uploading code coverage to codecov to github action

[jagerber48]

uncertainties has a codecov page. This should be connected to the new github action that runs tests during pull requests.

I believe the best way to do this is with some kind of secret token for the repo. I'm not sure if we'll need intervention from @lebigot on this or if the lmfit organization can make the connection to codecov independently.

[lebigot]

In the Codecov page of the project, I don't see any clean way of sharing rights or anything: in principle, I think I could share tokens, but then the Codecov page would still be for lebigot/uncertainties, which can be confusing given the fact that the repo now lives in the lmfit GitHub organization. I'm not sure what impact this can have on GitHub Actions.

So I guess that the best is for the lmfit repo to connect directly to Codecov? I'm not familiar with how this can be managed for a team of contributors, though. I would then deactivate Codecov for lebigot/uncertainties. What do you think?

[jagerber48]

Ok, I looked into this. As a member of the lmfit organization, I see that on my codecov page I can access my organization repos. So I think I should be able to add a token and get coverage uploading to codecov in an lmfit/uncertainties space that all organization members should be able to access. I will get a PR going when I get the chance. Probably in the next few days.

[jagerber48]

Closed by #202