Draft: Integrate in geant4_v10r7p3 the G4HepEm package

This MR integrates inside geant4 the functionalities of the package G4HepEm an R&D project by the geant4 collaboration to improve the computing performances of the EM shower generation covering e+/e- and gamma transport

changed milestone to %Geant4 v10r7p3t1

added new feature label

requested review from @gcorti and @witoldp

assigned to @veltri

restored source branch mveltri_g4hepem_integration

added 1 commit

• 29a96375 - Use the same histos and geant4 macro file as in G4SamplngCalo test

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added 1 commit

• 89d82d8a - fix some bug

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added 1 commit

• 110a5884 - Include compilation support for Geant4 >= 11.0

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changed milestone to %Geant4 v10r7p3t3