REQIF11 — Associate RelationGroup to source specification and target specification

In the RFC, a "RelationGroup" element is associated to "specRelations".

However, in contrast to prior versions of the Requirements Interchange Format,
the ReqIF RFC does not account for the fact that grouped relations should either link
requirements of two specifications (e.g. Customer Requirements Specification and
System Requirements Specification) or requirements within one specification.

Thus, two associations should be added to "RelationGroup", called "sourceSpecification" and
"targetSpecification" to point out the specifications in which the
source "SpecObject" elements and the target "SpecObject" elements connected by
the "specRelations" are located.

Note that having these associations in place would also greatly increase performance of ReqIF tool
implementations.

An additional constraint should be added to the RelationGroup section that defines that the "specObject" elements
connected by the "specRelations" must be referenced by a (SpecHierarchy element) of the respective specification.

Update the textual description and the diagrams shown in the specification, the
machine-readable file reqif.cmof and the XML schema file reqif.xsd to reflect the
change as described in the issue.
Also, update the textual description of SpecRelationType to clarify the distinction
to RelationGroup.