Adds code coverage through Codecov

[ilumsden]

This PR adds code coverage to our unit testing using Codecov.

[codecov-commenter]

Welcome to Codecov 🎉

Once you merge this PR into your default branch, you're all set! Codecov will compare coverage reports and display results in all future pull requests.

Thanks for integrating Codecov - We've got you covered ☂️

[ilumsden]

@pearce8 I've gotten the integration with codecov working, so this PR is now ready for review. You can see the codecov reports at the following links:

• develop branch (i.e., where the main coverage report can be found after merge): https://app.codecov.io/github/LLNL/thicket/tree/develop
• This PR: https://app.codecov.io/github/LLNL/thicket/tree/TauferLab%2Fthicket%3Acodecov

The badge in the README will show the coverage for the develop branch. As a result, it will not start working until after this PR is merged.

Finally, I've enabled Codecov's "comment" feature, which creates comments in PR like the one's above. For other PRs, these comments will explain how code coverage will change as a result of the PR. There is supposed to only be one message per PR. I think the multiple comment thing might just be a result of me messing around with codecov so much, so I personally don't think that's a problem at this point.