Add a section on Splitting Set Theorem and its consequences

[javier-romero]

This is motivated because now there is a gap between the definitions provided in the Introduction section and the explanations of the Modeling section.

The Easy ASP tutorial provides a foundation for the explanations of the Modeling section,
but the Introduction and the Easy ASP tutorial are completely disconnected.
The Splitting Set Theorem should help to fill this gap,
as one can see the Easy ASP interpretation as just a version of that theorem.