Implemented the execution of a custom __check__ function, when creating,...

... updating or saving a model

Details:
    User may define a __check__(self,data) method for Node-s and Relationship-s.
    Here the user may check the coherence of the supplied data. "data" is a dictionary of the defined properties.   This function has no return value but may raise an exception.

    The method is run when model is saved to the database (_create,_update,save).
    It is up to the user to be prepared for the exceptions.