Associate parent truth particle to measurements by index.

Currently the measurements converted to xAOD are associated to truth particles via the truth particle barcode. However, the latter is not unique in full truth pileup samples. Since all truth particles are added to a single xAOD container the truth particle index can be used as a unique identifier. Since the barcode serves as a second purpose to identify geant4 created truth particles the barcodes are kept and the indices are added as an additional decoration. The decoration is optional and can be turned off by not providing a truth link container.

This impacts the technical efficiency reported by InDetPhysValMonitoring when full truth pileup samples are used.