Fix PDG ID merging to exclude SUSY colored sparticles (squarks/gluinos)

Thanks to @ekuwertz and @zmarshal . Tagging @ljeanty @fmeloni @wfawcett @jmaurer @khoo @mhance @mcfayden @rurikova

NB: This affects MadGraph 2.6.2+ (where the default PDG IDs merging "bug" was introduced in 2.6.2).