GENE — abstract base classes should be removed from ref entity declarations in DTD

The DTD currently is generated with the rule that the *_ref entity
includes abstract class refs. Since these can't be instantiated,
implementations break when trying to figure out which class
to map to. This is because the identifier concept allows using
the identifier to be an alternative key in the case that the
identifier isn't immediately resolvable. But without knowing the
actually base class, this won't work. This is a particular
problem for use in disconnected inhouse high through-put pipelines.

Recommend that the abstract base classes be removed from the ref
entity declarations in the DTD.

Resolution of the issue would change:
MAGE-ML.dtd
(through a change to WriteDTDClassElement)
Section 4.1.2, "Mapping from MAGE-OM to MAGE-ML"

see below