Actually not able to check relation_to_type deletion constraints

The EmField version cannot be tested yet (drop column not yet implemented for sqlite in Database.sqlalter)

Also add a new function is sqlutils to execute ddl

 + pep8 and pylint

ignoring:
 - line too long
 - doctring

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