Interventions' attribute "variables" set to an empty list by methods sumOutVariable and maxOutVariable

[Manuel Luque]

The Interventions returned by methods sumOutVariable and maxOutVariable has an empty list value in the attribute "variables". Intervention is subclass of Potential and thus it should respect the rules of Potential: attribute "variables" should contain the list of variables that define the domain of the potential.

Please, be careful when correcting this issue: Intervention is a recursive tree-like structure; thus, any subtree should respect the rules for the attribute "variables".

If this issue is not corrected, then we could have unexpected results in the GUI and in the inference JUnit tests.