Python xAOD Robustness Update, master branch (2021.06.30.)

Similar to what xAOD::Init() does, I now updated xAODRootAccess.GenerateDVIterators to access the xAOD types in a specific order explicitly. This is meant to work around the issue described in ATLASG-1602, but not strictly speaking fix it unfortunately. 😦

Still, a workaround should be better than nothing for now...