Some clenup in the Generators, preparation for HepMC3 migration

Generators/AtlasHepMC/AtlasHepMC/GenParticle.h

Generators/AtlasHepMC/AtlasHepMC/SimpleVector_fwd.h

Generators/GenAnalysisTools/EventBoost/src/EventBoost.cxx

Generators/GenAnalysisTools/ReweightTools/CMakeLists.txt

Generators/GenInterfaces/CMakeLists.txt

Generators/GenInterfaces/GenInterfaces/GenInterfacesDict.h

Generators/HepMCAnalysis_i/src/PdfAnalysis.cxx

Generators/Hydjet_i/src/Hydjet_i.cxx

Generators/ParticleDecayer/src/ParticleDecayer.cxx

Generators/QGSJet_i/CMakeLists.txt

Generators/Starlight_i/src/Starlight_i.cxx

Generators/TruthIO/CMakeLists.txt

Generators/TruthUtils/CMakeLists.txt

Generators/TruthUtils/TruthUtils/HepMCHelpers.h

...csAnalysis/TruthParticleID/McParticleEvent/CMakeLists.txt

Reconstruction/MissingETEvent/CMakeLists.txt