Check license headers with reuse

[fholger]

This is a follow-up to #703. Closes #689.

This PR adds missing license and copyright information for all files (either by adding a license header or bulk-marking the files in .reuse/dep5 where license headers are not possible/undesired) and attempts to achieve full compliance with the reuse-tool. Reuse is now also called in the GitHub actions pipeline to ensure compliance going forward.

I am marking this PR as a draft because I am not entirely sure about the correct license for some of these files, but I hope that this PR can serve as a discussion on what the correct choices should be.