READY: Resolve "Enforce code format"

In discussion with all developers, we decided on a code style, reflected by clang-format. The push includes the following changes:

• clang-format is now mandatory for development and code must be properly formatted to be able to push to master
• A check if the code is properly formatted by the given .clang-format style file, is now part of the CI pipeline test
• The style file was tested on version 10 and 12 with the style file adhering to version 10.
• A new script scripts/check_format.sh has been added which is used by CI but can also be used by the user for two tasks:
  • Check if any files need to be formatted
  • Pass -r to reformat all files in place with clang-format
• All code has been reformatted to the decided style

Closes #121 (closed)