Add __cmp__ function for Atom class
Add __cmp__ function for Atom class

chemfiles/chemfiles,lscalfi/chemfiles,Luthaf/Chemharp,chemfiles/chemfiles,lscalfi/chemfiles,chemfiles/chemfiles,Luthaf/Chemharp,Luthaf/Chemharp,chemfiles/chemfiles,lscalfi/chemfiles,lscalfi/chemfiles