Error for derived predicates in initial state.

Previously, derived predicates in the initial state weren’t handled at
all. That’s not an issue, because derived predicates shouldn’t occur in
the initial state. However, as a defensive measure, this commit adds an
error thrown in such cases.

src/plasp/pddl/TranslatorASP.cpp

@@ -456,8 +456,7 @@ void TranslatorASP::translateInitialState() const
 		const auto handleDerivedPredicate =
 			[&](const ::pddl::normalizedAST::DerivedPredicatePointer &, bool = true)
 			{
-				// TODO: implement
-				//translatePredicateToVariable(m_outputStream, *predicate, true);
+				throw TranslatorException("derived predicates should not occur in initial state");
 			};
 
 		const auto handleNegatedDerivedPredicate =