Fix for broken HepMcParticleLinks seen during MDT_Digitization (ATLASSIM-4225)

An issue where HepMcParticleLinks from MDTSimHits were always invalid during pile-up digitization jobs was reported in ATLASSIM-4225. This was traced back to the fact that the HepMcParticleLinks were always pointing to whatever StoreGateSvc instance that MDTSimHit was read from rather than the output StoreGateSvc instance. This was a problem as in production configurations the input StoreGateSvcs are all cleared before the MDTSimHits are processed. By forcing the HepMcParticleLink::cptr() method to always look in the current output StoreGateSvc instance, it finds and links against the copy of the truth information held there. This is the same behaviour as in previous production campaigns and should fix ATLASSIM-4225.

This merge request also updates the log messages printed by HepMcParticleLink - if this requires updating too many log files I will move this to a separate merge request though.