Scikit-hep cookiecutter template

[rkansal47]

• Update repo to use the Scientific Python cookiecutter template
• HHbbVV can now be installed as a Python package
• Instructions in README
• More strict pre-commit checks and requirements (hence why nearly all the Python files have been updated)
• Test postprocessing and run.py locally
• Test condor submissions yet

The pre-commit requirements can be pretty annoying at times, so we can remove them... However, they have helped catch a lot of really subtle Python bugs so I thought they're worth trying.