Add Example Notebook for the Core

[mmschlk]

Add a tutorial/example for the ELC/EC and how it compares to the Shapley (and maybe Banzhaf) value. The notebook should be written in an accessible mannar but can of course walk through some formalism. It would be nice to see when the SV is in the Core or what it means that this is the case.

Of course the notebook should also show how the core can be computed with shapiq and what caveats exist (what the slack variable means and so on).