More attributes checks

[Ezibenroc]

Cf. ProjetPP/PPP-QuestionParsing-Grammatical#96
The implementation of the datamodel should enforce such things (throwing an AssertionError in this case for instance).

[progval]

The specification does not say predicates should be resources.

[Ezibenroc]

The specification at least say that only resources represent some raw data. For all the other elements, you should check that the objects passed as argument for the instantiation belong to the datamodel.

I opened an issue to fix the specification for the predicate.