Add `mypy` checks to `pre-commit` checks

[paddyroddy]

mypy is a great library which has a pre-commit hook. It allows you to enforce that python typing is obeyed (but would pass if no typing) i.e. a function expecting an int but receives a float would be a fail

I've checked how the repo is doing and at the moment it fails mypy big time

[paddyroddy]

working on this in fix-issue-84 branch