Enable pre-commit hooks

[connortann]

Plenty of PRs have failed jobs due to code formatting and flake8 errors, which suggests that some time is being wasted by devs having to run these tools manually.

We could use the pre-commit python package as a developer dependency, and add a .pre-commit-config.yaml file together with some instructions in the contributor guide to install.

This would mean yapf is run locally automatically on relevant files before each commit is made, so devs can spend their time on more important things.