Clang format checking

[DanielWielanek]

This is another attempt to set up automatic format checking of modified files.

• files are compared with the main branch, only changed files are checked (repo contains too many files to check all of them)
• files are not modified, to avoid a situation wherethe local repo is different the remote one

[veprbl]

Looks like a cool proposal. Perhaps, this should only apply to select whitelisted files and the new ones, because I don't think reformatting every touched file is what we want.

[DanielWielanek]

Hi, but then we should discuss what to put into this whitelist .

[JohnTheBlindMilkman]

I think it should be compliant with the STAR C++ Coding Guidelines (https://www.star.bnl.gov/public/comp/sofi/guidelines/codingguide.xml) and the naming standards (https://drupal.star.bnl.gov/STAR/comp/sofi/soft-n-libs/standards).

On that note, looking at e.g. the StHbtMaker, 90% of the code is indented with two spaces and is 80 columns wide.
If that is the case for other classes and makers, then if we set the clang-format to those guidelines, it shouldn't affect much of the codebase (correct me if I'm wrong).