fix in lib_tw from bug I introduced earlier

ess/lib_tw.py

@@ -212,7 +212,7 @@ class LATTICE:
                             except IndexError:
                                 pass
                     else:
-                        self.lst[i].apt = self.lst[i - 1].apt
+                        self.lst[i].apt = self.get_elem_idx(self.lst[i].idx_elem - 1).apt
             except AttributeError:
                 pass