Allow Generator wrappers to use lift with one new state

It's just a nicer interface than calling
`liftModifiedSolution(sol, InterfaceState{s}, InterfaceState{s}, child)` everywhere.

core/src/container.cpp

@@ -762,9 +762,16 @@ void ParallelContainerBase::liftModifiedSolution(SolutionBasePtr&& modified_solu
 void ParallelContainerBase::liftModifiedSolution(SolutionBasePtr&& new_solution, InterfaceState&& new_propagated_state, const SolutionBase& child_solution) {
 	assert(child_solution.creator());
 	assert(child_solution.creator()->parent() == this);
-	assert(pimpl()->requiredInterface() == PROPAGATE_FORWARDS || pimpl()->requiredInterface() == PROPAGATE_BACKWARDS);
 
-	pimpl()->liftSolution(std::move(new_solution), child_solution.start(), child_solution.end(), &new_propagated_state, &new_propagated_state);
+	if(pimpl()->requiredInterface() == GENERATE){
+		// in this case we need a second InterfaceState to move from
+		InterfaceState new_to{ new_propagated_state };
+		pimpl()->liftSolution(std::move(new_solution), child_solution.start(), child_solution.end(), &new_propagated_state, &new_to);
+	}
+	else {
+		// pass new_propagated_state as start *and* end. We know at most one will be used.
+		pimpl()->liftSolution(std::move(new_solution), child_solution.start(), child_solution.end(), &new_propagated_state, &new_propagated_state);
+	}
 }
 
 void ParallelContainerBase::liftModifiedSolution(SolutionBasePtr&& new_solution, InterfaceState&& new_from, InterfaceState&& new_to, const SolutionBase& child_solution) {