Enforce the formatting style

Summary

The WebKit formatting style has been chosen for this repository and its use is described in the contributing guide. The style should be enforced.

What is the current behavior?

Nothing enforces the formatting style.

What is the expected correct behavior?

The formatting style should be enforced.

Possible fixes

AFAIK, this cannot be enforced with pre-receive hooks on the CERN GitLab. The check can easily be implemented on a simple CI. However there is no reason to wait the CI for the checks: a pre-commit hook should be provided, so that the developers do not need to worry about the style.