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)

subordinates implementation is note tested

Not tested because of bugs in EmField

Changed comments about EmFieldGroup.fields()

It takes an EmComponent child class as parameter and return an sqlalchemy Table

Passe tous les tests

Récupération dans EmComponent directement

Suppression du init et passage des classes plutôt que les instances pour les fieldtypes dans le _fields

Now _fields and table MUST be class attribute.
Its important to note that _fields now consist in a list of (name, ftype) tuples and that ftype is a EmFieldType class and NOT an EmFieldType instance