docs: 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