Adding decoration with 4-momentum for SoftBVrtCluster vertices
Modified Reconstruction/VKalVrt/SoftBVrtClusterTool to decorate vertices with the 4-momentum obtained from the tracks attached to the vertex. The change is needed in view of importing the vertices into the DAOD_PHYS, where the tracks attached to the vertex will not necessarily be available to compute the vertex 4-momentum on the fly.