refactor generators to fix Coverity defects

Fixes:

in Generators/GeneratorObjectsTPCnv/src/initMcEventCollection.cxx:

• CID 17030: use move instead of copy
• Rewrite logic to avoid creating temporary pointers in loop

in Generators/TruthUtils/TruthUtils/AtlasPID.h:

• CID 17668: use const auto & instead of auto
• CID 17669: use const auto & instead of auto
• CID 17670: use const auto & instead of auto

in Generators/Herwig7_i/BB4LHerwig/BB4LPowhegLesHouchesFileReader.cc:

• CID 17676: use move instead of copy

in Generators/Hijing_i/src/Hijing.cxx:

• CID 17706: use move instead of copy