Migrate epos to HepMC3

In this MR:

• Migrate epos to HepMC3
• small fix to epos interface: check for the size of beams vector

This merge request affects 1 package:

• Generators/Epos_i

added Generators master review-pending-level-1 labels

CI Result SUCCESS (hash bbd1945b)

	Athena	AthSimulation	AnalysisBase	AthGeneration
externals
cmake
make
required tests
optional tests

Full details available on this CI monitor view
Athena: number of compilation errors 0, warnings 0
AthSimulation: number of compilation errors 0, warnings 0
AnalysisBase: number of compilation errors 0, warnings 0
AthGeneration: number of compilation errors 0, warnings 0
For experts only: Jenkins output [CI-MERGE-REQUEST-CC7 15562]

The changes are fine and there are no CI issues. Approving.

Jason (L1)

added review-approved label and removed review-pending-level-1 label

merged

mentioned in commit 9c5f4310

added sweep:ignore label