Add static type-based linting

[1kastner]

In the future, the command mypy --ignore-missing-imports --package conflowgen or an equivalent other static type checker should not find any errors anymore. As the code works fine in all known cases, we need to check how to deal with the failing instances. Is it better to add some assertions (especially, assert not None)? Should we maybe ignore a few lines?

[1kastner]

For the unittests, this is a bit problematic, see discussion at python/mypy#4063. We might also just exclude the unittests from the analysis? That might be better than using unittest asserts first and then use assert statements to state the very same thing! The tools we use might have some impact on our code but I do not want to make this project fit a linter if the adjustments lead to code duplications and the introduction of artifacts that just exist to satisfy mypy.