Wrong list of possible variables for a potential inside a tree/ADD

[Francisco Javier Díez]

The attached network has a potential for each branch in the tree/ADD for Cost [0]. When I click “Add variables to potential” for any of the two, the list of variables should be {Time in treatment [0], State [0]}, but it is {Therapy, Time in treatment [0]}.

[Manuel Luque]

This commit fixes this issue.