Fixed a minor performance lack impacting audited abstract entities
Added a toString() abstract method in AnswerValue, implemented in each
AnswerValue specialization
Added a custom cache on UnitCategoryService
Define configuration for caching (not working for now!)
Minor improvements in AbstractJPAPersistenceServiceImpl
Implemented QuestionSetAnswerService.findByParameters() method, adapted
other methods, adapted linked entities
Fixed problems related to removal and update of answers