Sum and Product as possible types of potentials for ordinary utility nodes

[Manuel Luque]

In this moment, the user can select "Sum" or "Product" as a type of potential for ordinary utility nodes (utility nodes whose parents are chance or decision nodes). I suspect this should not be allowed.

[Miguel Ángel Artaso Landa]

Modified the method 'validate' in the classes 'SumPotential' and 'ProductPotential' of the package 'org.openmarkov.core.model.network.potential' so the user cannot select 'Sum' or 'Product' as type of potential in value nodes that are not super-value-nodes.