TruthClustering fix for standard truth PU [ATLITKSW-158]
This MR addresses the issue raised in https://its.cern.ch/jira/browse/ATLITKSW-158 Only HS particles are counted as contributing and cases where no particle is found to contribute are treated identically to those where a single one contributes. The issue of duplicated tracks, with spurious pixel split hits and shared strip hits, is thus fixed.