Black: code formatting

Implements #94 (closed)

Uses python package pre-commit to call Black on every commit.

Needs to call pre-commit install once the repo is clone to install git hooks. Black is called on staged files. To call black on all files, use `pre-commit run --all-files'.

Commit should fail if black makes some changes.

I've added pre-commit and black in the Pipfile as dev-packages.