Reformat project with clang-format

• Add a clang-format file for the correct format to the project according to the guide lines in TODO.txt:
  • Klammern von Bloecken beginnen immer in neuer Zeile
  • einruecken von 4 Leerzeichen (kein Tab)
• Reformat the whole repo: Best when no other large works are in progress, as the merging then may result in a lot of conflicts
• (optional) Add check for correct format to CI