Mantain exact potentials when adding/removing links from utility nodes

[Iago París]

Instead of returning a utility table to uniform after every modification to the node. Make it behave like editions to chance nodes with table potentials: copying cells when adding links and removing cells when removing links.

That will avoid the students (among others) to refill by hand an utility table every time they modify the node containing it, which is a bit painstaking.

[Jorge Pérez]

Be careful, maintain the potential is not a trivial change. You need to ensure every potential type is working homogeneous.

[Jorge Pérez]

Please, check Issue #395.