Merge branch '121-enforce-code-format' into 'master'

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

See merge request hpc/gekkofs!66