AthSimulation: Some simple preparation to migration to HepMC3
This MR is a preparation to the migration of AthSimulation to HepMC3. The main goal of this MR is to reduce the number of files where the changes are needed, i.e. migrate to HepMC3 or version-independent code the files where the changes are just couple lines.
In this MR:
- some usage of auto
- use std:: namesapace explicitly
- c++ rage loop
- use free functions from HepMC:: namespace.