Add allAttributes method (similar to what UML offers)

[jcabot]

In UML, we have:

allAttributes() : Property [0..*]{ordered}
The query allAttributes gives an ordered set of all owned and inherited attributes of the Classifier.

All owned attributes appear before any inherited attributes, and the attributes inherited from any more specific parent
Classifier appear before those of any more general parent Classifier. However, if the Classifier has multiple
immediate parents, then the relative ordering of the sets of attributes from those parents is not defined.
body: attribute->asSequence()->union(parents()->asSequence().allAttributes())->select(p |
member->includes(p))->asOrderedSet()

We should have the same in the core of BESSER

[ivan-alfonso]

The method has been implemented (but without any ordering)