Add a license consent to the PR template

[ekatef]

Closes #24.

Changes proposed in this Pull Request

Add a consent to the GPLv3 for PRs. I suspect, there may be not crucial as the project is anyway licensed under GPLv3, but may be nice to have, anyway.

Checklist

• [x] I tested my contribution locally and it seems to work fine.
• [ ] Code and workflow changes are sufficiently documented.
• [ ] Newly introduced dependencies are added to the main environment at PyPSA-Earth repository
• [ ] Changes in configuration options are added in all of config.default.yaml and config.tutorial.yaml.
• [ ] Add a test config or line additions to test/ (note tests are changing the config.tutorial.yaml)
• [ ] Changes in configuration options are also documented in doc/configtables/*.csv and line references are adjusted in doc/configuration.rst and doc/tutorial.rst.
• [ ] A note for the release notes doc/release_notes.rst is amended in the format of previous release notes, including reference to the requested PR.

[davide-f]

Great :D many thanks!

[ekatef]

Great :D many thanks!

Thanks a lot to you! :D