Adding original track decoration for GSF Tracks to PHYSLITE

The decoration connecting the GSF track particle to the original track particle (in a different collection) was missing from PHYSLITE. This caused some issues in overlap removal in rare cases, and potentially other places (unclear about MET). I've checked that the links are there, seem to point to the correct container, and seem to have an index (I hope that's sufficient to indicate that this is working!).

Thanks very much to @mhank for the report!

Discussion: https://indico.cern.ch/event/1264717/contributions/5318949/attachments/2612345/4513767/amg_mh.pdf

Original thread: https://groups.cern.ch/group/atlas-sw-amg/Lists/Archive/Flat.aspx?RootFolder=%2Fgroup%2Fatlas%2Dsw%2Damg%2FLists%2FArchive%2FPotential%20PHYSLITE%20Issue%2D%20ElectronMuon%20OR&FolderCTID=0x012002009E45C4887AC2C0468B02F05418657558