C-FOVE: Propositionalization

- Added method to propositionalize SimpleParfactors
- Parfactor: added method to check if it is constant
- LogicalVariable: implemented method to get individuals satisfying
constraints
- Population: method to retrieve individuals as a set, not allowing
doubled individuals in constructor.
- SimpleParfactorTest: propositinalization test
- Pool: method to test propositionalization

Population class seems to be an overhead. It should be better modeled
or removed from the package.