Use mypy for static type checking

[regDaniel]

Describe the new feature

Currently mypy is not used in this project. For the scope of it, stability is of utmost importance and we should use a static type checker to avoid unnecessary TypeErrors at runtime.

[fpavogt]

Should this be linked to #121 (and closed when this PR gets merged) ? Or is the idea behind this Feature to have a more formal implementation of mypy ?

[regDaniel]

There's one other task still to follow: We'll add mypy to the linting actions workflow, we have an AMAROC ticket for that, which we will address in the next 3 weeks. Then we can close this issue.