[561825] Merge a Sirius Node doesn't merge "actualMapping" difference.

The implies/impliedBy relations of Sirius Nodes should not be
automatically merged and just noted as "MERGED". These differences
should be merged just like the others.

- Add a new Merger, SiriusReferenceChangeMerger, to manage these
"implies/impliedBy" relations.

- Add unit tests.

Bug: 561825
Change-Id: If191702dc31ec00cce0410ecc39a0ee3e5dac0dd
Signed-off-by: Glenn Plouhinec <glenn.plouhinec@obeo.fr>