fix implementation for attributs group/fields

In magento an attribut can be in various group (only one group per attribut set but this field can be in two different group in two different attribut set)

Basically we need to refactor the model and replace it by :

First we have to introduce an new object "field_group_position" that have for field "sequence", "field_id", "group_id", "attribut_set"
field_group_position
-many2one to group
-many2one to fields
-many2one to attributs set

attributs_set should have
- one2many to the group
- on22many to the field_group_position

group should have
- many2one to the attribut set
- one2many to the field_group_position

field should have
- one2many to field position

This should be done ASAP