Type hints for Python functions

[PicoCentauri]

Type hints are useful for new users to understand what functions are doing and also help during the development because IDE's and linters can use this information to help the developer.

Currentlty, many functions miss these type hints and we should add them.

[Luthaf]

If we add type hints, we should also add mypy (or another checker) to check that code using these types hint is correctly doing so (I don't think mypy can check if the type hints themselves are correct, although that would be great as well!)