Updates to AtlasHepMC to fix warnings in nightly builds (small MR)

This MR affects only HepMC3 branch.

• Define comparison operators to deal with failing google tests
• fix order of inclusion of HEPEVT related headers

This merge request affects 1 package:

• Generators/AtlasHepMC

This merge request affects 2 files:

• Generators/AtlasHepMC/AtlasHepMC/IO_HEPEVT.h
• Generators/AtlasHepMC/AtlasHepMC/Operators.h

added Generators master review-pending-level-1 labels

CI Result SUCCESS (hash a821a65a)

	Athena	AthSimulation	AthGeneration	AnalysisBase	AthAnalysis
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
AthGeneration: number of compilation errors 0, warnings 0
AnalysisBase: number of compilation errors 0, warnings 0
AthAnalysis: number of compilation errors 0, warnings 0
For experts only: Jenkins output [CI-MERGE-REQUEST-CC7 28846]

Code changes look fine, approving. Pienpen(L1)

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

merged

mentioned in commit 48312009

added sweep:ignore label