Maintaining an alias for the new EmComponent.db_engine

Conflicts:
	EditorialModel/components.py

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