Add type annotations so that the project can be checked using mypy

[Gaspi]

The mypy type checker can be run manually with mypy src.
The project can also be checked with ruff check ..

[Gaspi]

The following instruction could also be added to setup the extension.

To install the mypy vscode extension:

code-server --install-extension ms-python.mypy-type-checker

then access the Settings of the extension and set Mypy-type-checker: Cwd to ${nearestConfig}.