Check for specific version of clang-format

This alters CMake and the git hook to check specifically for clang-format version 4.0 instead of any >= 4.0. This should solve a few issues we have seen with inconsistent formatting across versions. I have also removed a few clang-format off tags from the code, which were only there to keep compatibility between versions.

I have not updated the manual with a specific version, I'm hoping to move to clang-format 6.0 soon and will then update the manual in the appropriate places.

Please observe that you need to re-run ./etc/git-hooks/install-hooks.sh in order to put the updated pre-commit hook in place.

This fixes #155 (closed)