Use `clang-format` in a CI job

[albinahlback]

Just something simple to check that the code is somewhat nicely formatted.

[edgarcosta]

We also discussed this in the last workshop, but I didn't get into it

[lgoettgens]

There also is git clang-format that, appropriate arguments, only formats those parts of those files that have been touched since the git merge-base of the PR. This would make it possible to gradually improve code formatting every time something gets touched anyway, in contrast to one gigantic PR that adapts all current files.

What certainly is missing, is a .clang-format config that resembles the wanted formatting style.