Add isort and pre-commit config

[jack89roberts]

• isort for sorting import order (https://pycqa.github.io/isort/index.html)
• pre-commit to auto-run isort, black and flake8 on commits
• also includes code conventions doc from Feature/349/convention docs #425

[jack89roberts]

Are you happy to use this @nbarlowATI ? Hadn't come across it before but spotted Angus using it in bpl-next and seems handy (sorts all imports alphabetically in sensible groups).

Also added a pre-commit config, again haven't done that before but means isort, black, flake8 run locally with every commit if you use it.

[nbarlowATI]

Good idea! I definitely like the idea of the automagic pre-commit stuff too!