Merge branch 'ATLASSIM5525' into 'master'

Update McEventCollectionCnv_p5 so that the input matches the output after a single round of T->P->T conversion

See merge request atlas/athena!49983

Generators/GeneratorObjectsTPCnv/src/McEventCollectionCnv_p5.cxx

@@ -606,7 +606,9 @@ McEventCollectionCnv_p5::createGenVertex( const McEventCollection_p5& persEvt,
 
   // handle the in-going (orphans) particles
   const unsigned int nPartsIn = persVtx.m_particlesIn.size();
-  for ( unsigned int i = 0; i != nPartsIn; ++i ) {
+  /// Note: the reversed order is because of ATLASSIM-5525
+  //for ( unsigned int i = 0; i != nPartsIn; ++i ) {
+  for ( int i = nPartsIn - 1; i >= 0; i-- ) {
     createGenParticle( persEvt.m_genParticles[persVtx.m_particlesIn[i]],
                        partToEndVtx,
                        datapools );

Generators/GeneratorObjectsTPCnv/test/McEventCollectionCnv_p5_test.cxx

@@ -223,19 +223,7 @@ void testit (const McEventCollection& trans1)
   cnv.transToPers (&trans1, &pers, log);
   McEventCollection trans2;
   cnv.persToTrans (&pers, &trans2, log);
-#if HEPMC3
   compare (trans1, trans2);
-#else
-  // TP conversion of HepMC2::GenEvents has a feature where the order
-  // of GenParticles associated with each GenVertex is flipped, so
-  // agreement is only restored after running TP conversion twice...
-  McEventCollection_p5 pers2;
-  cnv.transToPers (&trans2, &pers2, log);
-  McEventCollection trans3;
-  cnv.persToTrans (&pers2, &trans3, log);
-
-  compare (trans1, trans3);
-#endif
 }
 
 void test1()