Release v0.13.0

See also !117 (merged) what we have to do before each release. The date in the license files do not need to be updated at the moment.

@mnitsch can you please especially check if there are additional changes required in the Nix files?

Open TODOs after release:

• rebuild and re-upload Docker images for testing (Python dependencies!)
• tag the release
• archive the complete code incl. submodules and provide via Releases page / Wiki
• push python-ud3tn-utils and pyd3tn to PyPI
• check that the Docker publishing job completes successfully :-)