Update clang-format to 7 and use yapf 0.24.0 for Python files

After a private chat with @fwinkl I decided to have a look to clang-format-7.

It touches a lot the code in Gaudi, but I would say in a good way (e.g. fix some weird alignments, align comments, add comment to namespace closing brace, and much more).

I took the occasion to also fix the lookup of clang-format command to stick to a fixed version.

While working on it, I also switched from autopep8 to yapf for Python, mainly because it has an approach closer to that of clang-format (i.e. read, understand and rewrite).

Given the way the CI job works, this MR can be applied any moment, and the reformatting could go in a second step... or we add it to this MR.