Change signature of getXyz() methods in HasXyz interfaces

Current signatures have <? extends X>, for example:

List<? extends MethodInfo> getMethods()

This works fine for Clover's internal data structures (as there are subclasses), but it's not usable by developers using this API. It should be possible to have an assignment like:

List<HasMetrics> methodMetrics = classInfo.getMethods()

Fix: change to <? super X>.

Note: We may need to expose additional methods in interface. They could throw Exception("not implemented") or return nulls.