Add model validation

[jecisc]

We often find some incoherences in models depending on the parser used or the version of the parser.
It might be nice to add a validation system to the model doing some checks.

For example:

• Can we have stub entities in non stub entities? I think no but Anne has that in a model. We could add a validation about that
• We discussed once with Anne and Nicolas that stub classes with the same name should share the same instance. Maybe we could validate that? stubClassNames := model allClasses select: #isStub thenCollect: #mooseName. self assert: stubClassNames size = stubClassNames asSet size
• We could check that the number of non stub entities equals the number of all children of root entities + the number of root entities
• ...

We don't have to enable this check at the import, but it might still be nice to have the possibility and to use that in the tests of our importers