GeantThinningAlg - Fix muon truth thinning

As already mentioned in !79026 (merged), the Geant4 thinning of the TruthParticleContainer is totally random if muons are involved. The reason for that is that the truthParticleLInk of the Muon container is not against the full TruthParticles collection but against the MuonTruthParticles . The particle element index used to declare whether a truth particle is associated with a muon or not is completely random.

The most simple fix is to link the deriveed muonTruthParticle against the one in the fat container and then to retrieve that mother in the thinning alg.

Tagging: @jchapman, @jcatmore, @christos, @jmasik for the offline-sw-review-approved