Migrate CMSSW from HepMC2 to HepMC3

[mkirsano]

HepMC is a HEP event record used in CMSSW. HepMC2 is currently used.
HepMC3 is a completely rewritten version. Many methods are different from HepMC2. These two records can live together in one program. I have a partly working prototype (working at ~10% functionality, pythia8 only) in my private area.
The evolving plan follows

1. Cope with library structure. By default the HepMC3 libraries structures are lib64/libHepMC3.so->libHepMC3.so.3. CMSSW does not understand it, I have to copy this structure in CMSSW_12_3_0_pre2/external/slc7_amd64_gcc10/lib by hands. - done
2. Rewrite code created for ExternalDecays. Seems still to be needed for the photos++ package (tauola++?)
3. Create GenEventInfoProduct3 class similarly to GenEventInfoProduct. Some methods need rewriting. - done privately
4. Create HepMC3Product similarly to HepMCProduct - done privately
5. Rewrite the code where the HepMC record is fed in G4. Seems to be here (Siewyan):https://github.com/cms-sw/cmssw/blob/master/SimG4Core/Generators/src/Generator.cc#L112. Problem: SimG4Core compiled locally does not work. In contact with V. Ivanchenko
6. At some point a special CMSSW branch will be needed?

[cmsbuild]

A new Issue was created by @mkirsano Mikhail Kirsanov.

@Dr15Jones, @perrotta, @dpiparo, @makortel, @smuzaffar, @qliphy can you please review it and eventually sign/assign? Thanks.

cms-bot commands are listed here

[mkirsano]

assign generators

[cmsbuild]

New categories assigned: generators

@mkirsano,@alberto-sanchez,@SiewYan,@GurpreetSinghChahal,@Saptaparna,@agrohsje you have been requested to review this Pull request/Issue and eventually sign? Thanks

[mkirsano]

assign simulation

[cmsbuild]

New categories assigned: simulation

@mdhildreth,@civanch you have been requested to review this Pull request/Issue and eventually sign? Thanks

[mkirsano]

I solved the problem with library structure, preparing PR to add external

[mkirsano]

PR cmsdist #7655

[mkirsano]

PR to include hepmc3 merged

[mkirsano]

I found that it is impossible to include both pythia8 - HepMC2 and pythia8 - HepMC3 converters in the same code.
To have a possibility to switch I will have to create another plugin: Pythia8HepMC3Hadroniser

[civanch]

@mkirsano , do you make a survey of all places inside CMSSW, which require a change? Likely it will be the same situation - new alternative code will be required.