Copy some neutral changes fom the HepMC3part2 MR
This MR has the sole purpose to reduce the size of another MR which implements migration of AthGeneration to HepMC3 (WIP now).
- The updates of code are completely of semantic origin. These are neutral for HepMC2. The idea is that the code will look the same for HepMC3. In most this goal is reached with "auto".
- Some includes are added.
Edited by Andrii Verbytskyi