MDAnalysis atom selections of aromatic rings present in the ligand molecule.
        Takes:
            * ring * - index in self.ligrings dictionary
        Output:
            * ring_selection * - MDAnalysis Atom group

def get_ligand_ring_selection(self,ring):
        """MDAnalysis atom selections of aromatic rings present in the ligand molecule.
        Takes:
            * ring * - index in self.ligrings dictionary
        Output:
            * ring_selection * - MDAnalysis Atom group"""
        ring_names = ""
        for atom in self.ligrings[ring]:
            ring_names = ring_names+" "+str(atom)
        ring_selection = self.topology_data.universe.ligand.select_atoms("name "+ring_names)
        return ring_selection