Continue adding static typing

[dfurtado]

I've already started adding static type checking but there are still some places that are missing.

The idea of adding static type checking is that the code will be self-documented, making it easy to contributors get up to speed and start working on adding new features to the framework. Also, it will ensure that the correct values are being passed to the functions and methods, helping us during development.

For more information about optional static typing, check: http://mypy-lang.org/