Tag source files with license comment

**do NOT merge yet**

Updates the source files with a header comment that includes the copyright and reference to the `LICENSE` file. I added a test that will be run with smoke tests to verify the copyright exists in all source files. This will alert us when new files are added and do not include the comment.

The test also checks the year, so we can expect it to fail on the first of each new year. I have not included any scripts to automate the process of updating the copyright each year. I think we can do that later... but, I'm open to other suggestions.

### TODO
* [ ] test pass?