Issue 128: Destruction of features

Now that all classes have been reviewed, one more conservative change request
appears:

E6 Destruction. P13 destroyed (was destroyed by): E19 Physical Object

should be changed to:

E6 Destruction. P13 destroyed (was destroyed by): E18 Physical Stuff

Pro: Features can be destroyed, like faces of statues, caves by earthquake etc.
Con: Destruction of a feature is a modification of the object. No need for this change.
Pro: It is counterintuitive and lacking precision to describe destruction of Features on the surface of earth as modification of the whole planet.

Both changes are conservative (monotonic) in the sense that no current data in
a CRM implementation is invalidated by that. Both changes have the agreement of
the CRM-SIG Steering Group.