Add GPLv3 license file

[nemobis]

The individual files already contain headers with the GPLv3 license statement and the list of authors, but for better clarity it would be nice to add the GPLv3 text in a LICENSE file in the repository root.

See instructions:
https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/adding-a-license-to-a-repository