Support for algorithms that rely on the hierarchy of the feature model

[tthuem]

While most sampling algorithms only consume the feature model as a propositional formula, there are a few algorithms that do also incorporate the feature hierarchy of the feature diagram. Not all benchmark models are useful in those cases (i.e., those being imported from DIMACS do not have a hierachy). Also, we might need a special input format or at least a separate category for comparison of outputs.