Copyright notice in file header

[tigercosmos]

Per #683 (comment), we currently only allow one name in the copyright notice, which does not accurately reflect the actual contribution of the file. I propose that we can have multiple authors in a file.

[yungyuc]

Thank you for bringing it up, @tigercosmos . The purpose of the copyright notice in the file header is to remind of the license, which requires to have a copyright holder. The convention was to have only one copyright holder in the header file.

I did not think it was important to accurately list all contributors in the file header, since the git history should suffice. But is does not hurt to be rigorous.

If we want to be rigorous, a clear rule of listing authors should be written first.