Rudder

Parent PR did the main refactoring and it is a good base for an enhanced model for our property hierarchy.

It can be further simplified and the naming still need to be adapted to the new structure:

- ParentProperty is not really a parent property, since it has a case for nodes which are always leaf. Plus, it creates ambiguity about what is (or is not) the "prop" in the PropertyHierarchy
- PropertyHierarchy is useless now, since we do have the property hierarchy in the ParentProperty object
- the semi-lattice structure is not exhibited, and we don't have anymore the linearization to top, which is the main intersting property for us

So we need to delete PropertyHierarchy, rename ParentProperty to something more generic, and implements/check semi-lattice properties, esp the linearization.