Adding offset in MergeSimSources breaks symmetry assumed by truth matching
When the offset is applied to the TrackID when using the LArSim module MergeSimSources the offset is applied both to the primary track Id and the "rolled up" track Id that is the negative of the original track Id (see https://github.com/LArSoft/larsim/blob/b45eb9ed60d76c792ba445784c457ad048f515fd/larsim/MergeSimSources/MergeSimSources.cxx#L228). For example a muon with Id 1 the delta rays will typically be assigned an Id of -1. When an offset of 1000 is applied these are changed to 1001 and 999 respectively, rather than the 1001 and -1001 one would expect. This breaks a symmetry that is assumed by many truth matching utilities (e.g. https://github.com/LArSoft/larsim/blob/68017a1f0e7f41c444973ae340a003dd41b77aa3/larsim/Utils/TruthMatchUtils.cc#L179) meaning these functions can return Ids that match to non-existent particles.