ACTFW-114 HepMC reader/writer

The HepMC format is a commonly used data format for the storage of Monte Carlo runs produced by MC generators like Pythia, Sherpa and Herwig. In order to use simulated events in ACTS, this MR considers a reader and writer of the HepMC file format and a translation of this data type into the ACTS data types and vice versa. The current version has the basic functionality (i.e. storing all necessary data). This storage directly depends on the particle and vertex storage in ACTS. So, a change in those data types is directly link with an adaption of the content of this MR.