Clang Format

[Yurlungur]

I think it's about time we formatted the repository, as it's getting pretty messy. This PR does the following:

• I implement a simple script in bash/scripts/format.sh, which queries git for every cpp and hpp file in git history, excluding submodules, and calls clang-format on all of them. Note a file must be committed to git for this script to format it.
• I add a github workflow which checks for formatting on PRs. It will fail if another run of clang-format doesn't match.
• I format the repo.

What do you all think? Are you happy with adding the workflow? Are you happy with the massive format commit? Does the script make sense to you? If you're happy, I think this should be a quick merge.

[Yurlungur]

There's currently a github actions outage, so the CI isn't triggering. Hopefully that will recover soon.

[Yurlungur]

@brryan I think I've addressed all comments. I now:

• Pin to clang-format-12 (checked that this is actually installable on both my linux boxes and available on Darwin. Hopefully this means it works for you too.)
• Modify the format.sh script so that it warns if clang-format is the wrong version. You can also point it to a specific binary with a command line variable. It also now errors out if there's no clang-format available.
• Reformatted
• Fixed the issues that were causing the tests to fail... I believe having to do with the test_utils.hpp header.