Eclasses generated in src-gen should all be generated as abstract

[opcoach]

It would avoid to set default implementations in src interfaces as these interfaces are implemented by the EMF generated classes.
As the generated EMF class are never used directly (only src classes will be instantiated), it is possible to set them as abstract.

Should change the isAbstract in GMAGenClassImpl.

On the other hand, packages, factories, util classes should not be abstract -> Must check that.