v0.1pre06: Upgrade to HepMC3, Compatibility with podio v00-15

What's Changed

• Upgrade to HepMC3 by @vvolkl in #9
• Adapt to new (v00-15) podio generated class names by @vvolkl in #10
• [ci] update centos7 docker image url by @vvolkl in #11

Full Changelog: v0.1pre05...v0.1pre06