Description

Adds a CODEOWNERS file.

Subdirectories and files are listed alongside users and PRs that modify files that match these patterns (either subdirectories or individual files) have to be approved by one of the listed GitHub users in order to be merged. It is necessary to have more than one person listed for any given set of files, since users cannot approve their own pull requests, and in case someone is unavailable or at a conference, etc.

See docs: https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-code-owners#codeowners-and-branch-protection

Related issues

Closes #747.

Checklist

Before this pull request can be reviewed, all of these tasks should be completed. Denote completed tasks with an x inside the square brackets [ ] in the Markdown source below:

• I have added a description (see above).
• I have added a link to any related issues see (see above).
• I have read the Contributing Guide.
• I have added tests for any new physics that this PR adds to the code.
• I have tested this PR on my local computer and all tests pass.
• I have manually triggered the GPU tests with the magic comment /azp run.
• I have requested a reviewer for this PR.