Introduce a class to convert AOD events to HepMC

The class `o2::eventgen::AODToHepMC` converts events stored in AOD
tables to `HepMC3::GenEvent` objects.   The objects are kept in-memory
so that clients (e.g., a future `o2::analysis::RivetWrapper` or similar)
can pick up that event structure and process it.

The converter class will convert `o2::aod::McCollisions` and
`o2::aod::McParticles` information, as well as auxiliary information in
`o2::aod::HepMCXSections`, `o2::aod::HepMC::PdfInfos`, and
`o2::aod::HeavIons`.

The built `HepMC3::GenEvent` data structures can optionally be stored on
disk in HepMC3 ASCII files.

The application `o2-aod-mc-to-hepmc` uses this converter class to
convert AOD files to HepMC files.

Note that the converter _only_ depends on
`Framework/AnalysisDataFormats.h` and not any other AOD information.

Compilation of `Generators/src/AODToHepMC.cxx` is contingent on HepMC3
being found at configure time.

The code is heavily documented and commented.

Author: cholmcc

Generators/CMakeLists.txt

@@ -50,6 +50,7 @@ o2_add_library(Generators
                        $<$<BOOL:${pythia_FOUND}>:src/DecayerPythia8Param.cxx>
                        $<$<BOOL:${HepMC3_FOUND}>:src/GeneratorHepMC.cxx>
                        $<$<BOOL:${HepMC3_FOUND}>:src/GeneratorHepMCParam.cxx>
+                       $<$<BOOL:${HepMC3_FOUND}>:src/AODToHepMC.cxx>
                PUBLIC_LINK_LIBRARIES FairRoot::Base O2::SimConfig O2::CommonUtils O2::DetectorsBase O2::ZDCBase
                                      O2::SimulationDataFormat ${pythia6Target} ${pythiaTarget} ${hepmcTarget}
                                      FairRoot::Gen
@@ -106,6 +107,7 @@ endif()
 if(HepMC3_FOUND)
   list(APPEND headers include/Generators/GeneratorHepMC.h)
   list(APPEND headers include/Generators/GeneratorHepMCParam.h)
+  list(APPEND headers include/Generators/AODToHepMC.h)
 endif()
 
 o2_target_root_dictionary(Generators HEADERS ${headers})