Adding type-hints, part 1

[ADBond]

Mostly adding a bunch of type hints to the code base.

This gets rid of approximately half of the mypy errors we get if we include the flag disallow_incomplete_defs, and thus is a stepping stone to include that check in our config.

The only noteworthy changes:

• Added a non-None default for estimate_u_with_random_sampling, as per linker.estimate_u_using_random_sampling fails with default arguments, with no clear indication why #2165 - needed some solution to avoid breaking mypy checks, but we can change this again in follow-ups
• ComparisonLevel has a (read-only) property comparison_vector_value - this is just a wrapper around the internal _comparison_vector_value, that does not allow the None option we have before it is associated with Comparison. This is mostly what we want to use anyhow

[RobinL]

Thanks! Sorry for the merge problem, was just about to fix it but you've done it!

[ADBond]

Thanks! Sorry for the merge problem, was just about to fix it but you've done it!

No worries - your PRs have been there a while so happy to sort out the merge - nothing major anyhow. Cheers!