Clang formatting

Add google style compliant .clang-format file to idg, do all the formatting and check compliance in CI. Code related changes are only induced by the clang formatter, and therefore purely esthetical.

Also, I enforce the gitlab-runner to pull ubuntu 20.04 for every job so that gitlab-runner is guaranteed to use the docker executor (I noticed that previously the shell executor was used for certain jobs). We may want to discuss the intended behavior (i.e. which executor is used) of the gitlab-runner a bit further.

To whom should I assign this MR, @tol or @veenboer ?