Simplify Child Relationships
In the current feature model, there are two types of ChildRelations:
FeatureGroup able to reference subfeatures and to specify their cardinality (min / max)
SimpleFeature able to reference only on sub feature
The SimpleFeature is redundant and can also be represented by a FeatureGroup with a cardinality min:1, max:1.
In addition, it is confusing when to use which element.
To optimize this, remove SimpleFeature and provide only FeatureGroup. Furthermore, the additional indirection with the abstract ChildRelation can be removed.
Will not be fixed because of investigation into the EMF feature model project