Fix/update license file and code file headers

[pseewald]

• full GPL3.0 license file needs to be included: https://choosealicense.com/licenses/gpl-3.0/
• Copyright header and year needs to be updated. I suggest:

 #    This file is part of fprettify.
 #    Copyright (C) 2016-2019 Patrick Seewald, CP2K developers group
 #    Copyright (C) 2019-2026 The fprettify contributors

@gnikit @awvwgk is there any need or convention to mention @fortran-lang? In my understanding it shouldn't be mentioned as the copyright lies with the authors, not with any organization. Ideally CP2K developers group shouldn't be mentioned either (nor should my name be mentioned), but we can't change that in retrospect.

[kc9jud]

I'd suggest using the SPDX format: https://spdx.dev/learn/handling-license-info/