bug #4361 :  Fix the bug about the position duplication within a single classification. The fix concerns both the javascript front-end of the classification on the PdC and in the JPA entities implied in the definition of a predefined classification on the PdC. For latter, a bean validation constraint (UniquePositions) is added but Hibernate 3.6.10 (or Spring-data) has some bugs in the validation of constraints when saving an entity.