Consider tightening data relationship source & target operation return types

[NicolasRouquette]

Instead of:

abstract class DataRelationship extends Term, DirectedBinaryRelationshipKind {
        ...
    op Datatype[1] target()
}

class EntityScalarDataProperty extends DataRelationship, DataRelationshipFromEntity, DataRelationshipToScalar {
        ...
    op Datatype[1] target() { range }
}

class ScalarDataProperty extends DataRelationship, DataRelationshipFromStructure, DataRelationshipToScalar {
        ...
    op Datatype[1] target() { range }
}
class EntityStructuredDataProperty extends DataRelationship, DataRelationshipFromEntity, DataRelationshipToStructure {
    ...
    op Datatype[1] target() { range }
}
class StructuredDataProperty extends DataRelationship, DataRelationshipFromStructure, DataRelationshipToStructure {
    ...
    op Datatype[1] target() { range }
}

Consider replacing the return type with the most specific DataType appropriate:

abstract class DataRelationship extends Term, DirectedBinaryRelationshipKind {
        ...
    op Datatype[1] target()

}

class EntityScalarDataProperty extends DataRelationship, DataRelationshipFromEntity, DataRelationshipToScalar {
        ...
    op DataRange[1] target() { range }
}

class ScalarDataProperty extends DataRelationship, DataRelationshipFromStructure, DataRelationshipToScalar {
        ...
    op DataRange[1] target() { range }
}
class EntityStructuredDataProperty extends DataRelationship, DataRelationshipFromEntity, DataRelationshipToStructure {
    ...
    op Structure[1] target() { range }
}
class StructuredDataProperty extends DataRelationship, DataRelationshipFromStructure, DataRelationshipToStructure {
    ...
    op Structure[1] target() { range }
}

[NicolasRouquette]

Ditto for source.