For immutable classes, immutability of superclass is checked, too. This
means in particular: (1) JicDetector: abstract classes can be annotated
as immutable, too. (2) ClassHelper: getNonPrivateNonStaticMethods()
replaced by getConcreteNonPrivateNonStaticMethods(). (3) ClassAnalyzer:
isImmutable() recursively checks immutability of superclass.