add CODEOWNERS file

[BenWibking]

Describe the proposal
GitHub allows the use of a CODEOWNERS file that specifies whose approval is required for PRs that modify certain specified files. Those people will be automatically assigned as reviewers of PRs that modify those files, and their approval is required for that PR to be approved.

This is useful to ensure that PRs that modify certain parts of the code are reviewed by the appropriate people. Many parts of the code have esoteric details that are not well documented, and it could be dangerous to allow anyone to approve PRs for parts of the code they are not an expert in.

Describe alternatives you've considered
Keep the PR review process as-is.

Additional context
Athena++ CODEOWNERS file: https://github.com/PrincetonUniversity/athena/blob/master/.github/CODEOWNERS

[BenWibking]

For instance, I think it would make sense for me to the be the "code owner" for everything in src/hydro/* and @chongchonghe to be the code owner for everything in src/radiation/*.

[BenWibking]

@psharda should be the code owner for everything in src/chemistry/*.