[Feat] Add type hints

[fjosw]

As a first step towards adding type hints, I added automatic type hints with monkeytype. Hints for the more complicated methods will need additional manual work but for the simpler methods this seems to have worked quite well.

[fjosw]

Not quite done but already in decent shape for a review @s-kuberski @jkuhl-uni:

• Most of the changes are just type annotations
• I changed a few lines to help mypy with understanding the type annotations
• I fixed a few smaller bugs in which explicit type checks were not consistent or did not cover certain edge cases

For now I ignored the input/* modules and most of correlators.py, the remaining modules only raise a few smaller mypy errors which I wasn't able to fix right away.

[s-kuberski]

Great, thanks a lot for all of your work! I'll have a look in the upcoming week.