code: def is_het_residue(self, obres):
          
          if not obres.GetResidueProperty(0):
              # If the residue is NOT amino (0)
              # It can be amino_nucleo, coenzme, ion, nucleo, protein, purine, pyrimidine, solvent
              # In these cases, it is a ligand candidate
              return True
          else:
              # Here, the residue is classified as amino
              # Amino acids can still be ligands, so we check for HETATM entries
              # Only residues with at least one HETATM entry are processed as ligands
              het_atoms = []
              for atm in pybel.ob.OBResidueAtomIter(obres):
                  het_atoms.append(obres.IsHetAtom(atm))
              if True in het_atoms:
                  return True
          return False
docs: Given an OBResidue, determines if the residue is indeed a possible ligand
          in the PDB file