WIP : Convert VPLightCluster to not use vector<LHCb::ID> FollowUp LHCb !1146
Follow up on LHCb merge request LHCb!1146 (closed) Adapt code to not use VPCluster::pixels()
Follow up on LHCb merge request LHCb!1146 (closed) Adapt code to not use VPCluster::pixels()