Wrong Instability for Abstract classes with inheritance classes

# Bug report

PhpMetrics 3.0.0rc6

If I understand correctly, Instability counts incorrectly for abstract classes with inheritors. Since the classes are abstract, we get `Affererent Coupling = 0`, even though the class has descendants that are explicitly linked to the abstract class. Because of this, abstract classes are placed in the zone of useless classes.

Or am I wrong?