helpful mypy type annotation checking

[richard-lane]

At the moment most things are type annotated, but there's no checks for consistency or that the type hinting is complete.

We want to:

• Create a mypy config file which:
• Checks that the type annotations in the code we've written are consistent with each other and match the actual return type
• Doesn't check for unecessary things that are annoying
• Make it work on the existing code
• Add a github action (in the linting step) that performs the type checking automatically on every push