Adds GenEvent.from_hepevt

[HDembinski]

This is the more flexible equivalent of fill_genevent_from_hepevt which used to be part of pyhepmc-ng but was removed in the switch to v2.0. It does not use the corresponding HepMC3 code, which is a mess and only works for an original HepEvt common block.

[coveralls]

Pull Request Test Coverage Report for Build 2898279593

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 91.525%

---

Totals
Change from base Build 2895297867:	0.0%
Covered Lines:	108
Relevant Lines:	118

---

💛 - Coveralls