FUML13 — Reference::equal should use Object::equals

The Reference::equals operation checks that two References are equal if they reference the same Object. This check is done by testing the identity of the two referenced Objects using "==". It would be better if the test on the Objects was done using the Object::equals operation. Even though the Object::equals operation itself simply uses "==", using Object::equals in Reference::equals allows for the possibility of subclasses of Object to redefine equals as used in Reference (this is desirable, for example, for the PSSM DoActivityContextObject class, which should be considered "equal", but not "identical", to the Object that it wraps).

Update Reference::equals and add Object::equals

Actually, Object does not currently override the equals operation, so that the CompoundValue::equals operation is inherited, which specifies equality based on value, not identity. However, if an appropriate Object::equals operation is also added, then the Reference::equals operation may be updated as suggested in the issue description.