Add type hints for type safety

[sadielbartholomew]

Add type hints across the codebase. They are valid syntax for Python v..3.5+, so are safe to add given we are now 3.7+.

We can then use type checking tools, such as mypy, to ensure robustness of our code and catch bugs relating to errors due to incorrect types.

See #785 (comment) for brief further discussion.