QVT — RelationalTransformation as a subclass of Transformation

To avoid dependency of QVTBase to QVTRelation due to the fact
that a relational transformation contains QVTRelation::Keys
Instances, the Transformation metaclass should be subtyped
within the QVTRelations package.
By the way, the association between relational transformations
and Keys is missing in the metamodel and need to be added.

Suggestion: Add a RelationalTransformation metaclass inheriting
from Transformation and add the missing association:
'RelationalTransformation::key : [*] Key

'
Correct the type of OperationalTransformation:refined so that
it is a RelationalTransformation (instead of 'Transformation')

(1) In section 7.11.3 insert the definition of the "RelationTransformation"
class with the following description:
"""
A RelationalTransformation is a specialization of a Transformation
representing a transformation definition that uses the QVT-Relation
formalism.
Superclasses
Transformation (from BaseQVT)
Associations
key : [*] Key

The keys defined within this transformation.
"""
(2) In Figure 7.7 inserts the class "RelationTransformation" inheriting
from "Transformation" and adds the link to the "Key" class