Migrate hijing to HepMC3.

In this MR:

• Migrate hijing to HepMC3
• attempt to optimize interface init a bit

This merge request affects 1 package:

• Generators/Hijing_i

added Generators master review-pending-level-1 labels

CI Result SUCCESS (hash 64b87c12)

	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 15563]

The changes look 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 3aa5a7fe

added sweep:ignore label