Fixes #6050

[greglandrum]

We need to keep track of the group IDs of the stereo groups as the molecule is being built.

[bp-kelley]

using std::find I think would make this code simpler.

   auto index = std::find(sgTracker.start(), sgTracker.end(), group_id);
   if( index != sgTracker.end()) {
         auto gAtoms = mol_stereo_groups[*index].getAtoms();
         gAtoms.insert(gAtoms.end(), atoms.begin(), atoms.end());
         mol_stereo_groups[*index] =
             StereoGroup(mol_stereo_groups[*index].getGroupType(), std::move(gAtoms));
   } else {
      // not seen this before, create a new stereogroup
       mol_stereo_groups.emplace_back(group_type, std::move(atoms));
       sgTracker.resize(mol_stereo_groups.size());
       sgTracker.back() = group_id;
      mol.setProp(cxsgTracker, sgTracker);
   }

[bp-kelley]

Should sgTracker be part of the StereoGroup object? This would remove the need for the implicit copy here.

[greglandrum]

I don't think the IDs should be part of the stereogroups. They only important here when parsing and can then simply be added when writing the groups out (which may not be in the order in which they were read or stored).