PSUM — Explicit Artifact Citation Element no longer available in SACM 2.3

Unfortunately SACM removed the explicit Artifact Citation element at some point in its revision history, which breaks backward compatibility. A solution for the use of citation in the PSUM Evidence and Risk package would be a specialization of the SACM ArtifactReference metaclass, then subsetting the reference pointing to referenced Artifacts to point to the cited Artifact

Resolve connection to SACM 2.3

A revision of SACM had removed the explicit citation modeling. To overcome this backward-incompatible change in SACM, the PSUM submission substituted the missing ArtifactElementCitation element in a stub model. This resolution corrects the situation by moving ArtifactElementCitation to the PSUM Evidence and Risk package, and make it specializing ArtifactReference of SACM. The change is purely in the PSUM Metamodel, but will also affect Clause 13 and Figures 8.1 and 13.1