SusyNtMaker::fillTruthParticleVars() adds ttbar truth for PowHeg+Pythia

Need to have the 4 momenta of ttbar for PowHeg+Pythia ttbar samples in order to perform re-weighting as 
described in: https://svnweb.cern.ch/trac/atlasoff/browser/PhysicsAnalysis/SUSYPhys/SUSYTools/trunk/Root/SUSYObjDef.cxx#L2833