Add GPL v3 License

We want to license community-solutions to ensure that there are no legal complications in the future. This makes sure that other FVs are fully legally allowed to continue to use community-solutions, and also opens up the possibility of using it outside of ETH (e.g. at UZH or EPFL), should the demand exist eventually.

Steps to do:

• Get approval from the Vorstand of VIS
• Make sure the legal situation is clarified
• Get approval from all contributors that made copyrightable contributions (these are all the ones that have 50+ lines in the master branch, might need to expand the list)
  • Lukas Möller
  • Benjamin Schmid
  • Philippe Voinov
  • Michael Keller (contacted via Slack)
  • Samuel Müller (contacted via mail)
  • Jacques Hoffmann (contacted via Discord)
  • Mark Csurgay (contacted via Discord)
  • David Bimmler (contacted via mail)
  • Jonas Konrath (contacted via Discord)
  • Thomas Gassmann (@tgassmann)
  • Julian Steinmann
• Merge!