add licence information to all scipts and files (MR !14)

• add GPL v3 license information to the repo and all files
• use project name FluxProGenie
• copyright is with MPI-BGC, and authors/contributors are listed in the AUTHORS.md
  • ## Copyright (C) 2025 Max Planck Institute for Biogeochemistry (MPI-BGC), Jena, Germany
  • ## Authors and Contributors: see AUTHORS.md
• the year of the copyright is 2025, assuming final changes for publication in this year
• does not change any actual code
• no testing is done

Closes #25 (closed)