Update potentials when adding/removing a link

[Jorge Pérez]

It must be revised how to update the potentials (if possible) and how to inform the user that the potential will be changed.

This revision requires a deep study of the possible relationships between nodes (e.g. a decision node that links a chance node with a restriction), and also how the different types of potentials must be updated in each of these scenarios.

Finally, we must decide how to inform the user that the potential is going to change (i.e. with a warning dialog, with a permission request, …).