We have to create a special fieldtype to handle this field

Note : the check method does not correct the EmFields only displays a warning

The me.json was not up to date (Models don't forward classtypes modifications to loaded json file)
Some bugfix on fieldtypes that was not used (because of old me.json)

This commit is full of bugs (in LeRelation etc.)

Begin to work on fieldtypes

Now on load default fields for EmClass are created. The EmClass.check method create them if they are not present.

Added default fields for EmClass, they are listed by classtype in EmClassType
Fixing some tests (testing that a new EmClass has no fieldgroups)
Fixing the delete_check of EmClass. Now an EmClass can be deleted if it contains only default fieldgroup and default fields

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)