Add a pre-commit hook

This MR adds a pre-commit hook.

The point of this hook is to avoid pushing commits that will trigger silly CI failures, to save CI runs and have a faster feedback loop (by having local feedback as opposed to CI feedback). To be fast the hook is incomplete: it doesn't check everything that gitlab CI will check; but it is correct: what the hook checks is required for gitlab CI to pass. The hook must be fast so that git commit stays fast.

This hook:

• executes python tests of staged *.py test files The point is to execute a subset of pytest that is required for a commit to pass CI. This is most useful when working on the python tests themselves.
• lints and typechecks staged *.py files.
• calls ocamlformat on staged *.ml, *.mli files in a way faster than make fmt. If staged files are being inspected (i.e. if --unstaged is not passed, see below), formatted files are added before creating the commit; hereby autoformatting the files upon committing.

The first two points are blocking: if a pytest call fails the hook returns a non-zero exit code, forbidding to commit. The same goes for *.py files that do not pass linting. The third point is not blocking, because the hook does not try to format a partially staged file (because the call to ocamlformat doesn't distinguish between staged and unstaged parts of the file). It seemed overkill to forbid a commit because some unstaged parts are not formatted.

The hook is installed as follows: ln -sr tests_python/pre_commit.py .git/hooks/pre-commit (rm .git/hooks/pre-commit if you have a hook installed already)

You can call the hook manually on modified but not yet staged files to make sure an upcoming call to git commit will succeed using the --unstaged flag: ./tests_python/scripts/pre_commit.py --unstaged.

You can pass --lint-only to avoid calling pytest, so that the hook is always fast. In this case, install it as follows:

cd .git/hooks
echo '#!/usr/bin/env bash' > pre-commit
echo './tests_python/scripts/pre_commit.py --lint-only "$@"' >> pre-commit
chmod +x pre-commit

I modified the documentation to mention the hook although I'm not really satisfied with the modifications. I feel this hook could be better advertised but I didn't find a better way.