Implemented EmType hierarchy, added utility function to classtype, changed pk for em_type_hierarchy table
EmType hierarchy is handled by 3 functions :
- EmType.superiors()
- EmType.subordinates()
- EmType.add_superior(em_type, nature)
- EmType.del_superior(em_type, nature)
Hierarchy implementation is not tested (because their is no tests...)
Added a function to get authorized relation nature from an EmClassType
Added the nature of a relation to the primary key of em_type_hierarchy table (triple pk : sup_id, sub_id, nature)