TauTruthMatchingTool: ISTruthMatched recomputed for derevations where decoration is not present

Closes ATLTAU-1769

MR to run on DAOD_PHYS removed necessity of IsTurthMatched in decorations, but introduced ElementLink warnings which should not have been seen otherwise. This was caused by checking if truthParticleLink was available as a replacement to checking IsTurthMatched. These are not identical as truthParticleLink is always available once a TauJet passes through truth matching.

Solution recomputes IsTurthMatched on the fly from the value of truthParticleLink, which allows us to keep the previous if statement logic. IsTruthMatched could be removed from code altogether in future due to this redundancy.