Change wording of license statement

This MR changes the wording of the license statement at the top of every file. It also updates the license checker to enforce the new wording:

- // Copyright (C) 2016-2018 Acts project team
+ // Copyright (C) 2016-2018 CERN for the benefit of the Acts project

This currently is rebased on !572 (merged), but on top of it it ONLY changes these license statements, and the script, nothing else.