refactor id_str() usage for finding altloc

mmtbx/validation/rna_validate.py

@@ -441,18 +441,17 @@ def show_summary(self, out=sys.stdout, prefix=""):
 
   def local_altloc_from_atoms(self, residue_1_deoxy_ribo_atom_dict, residue_1_c1p_outbound_atom, residue_2_p_atom):
     #conformer.altloc masks whether a residue has true alternate conformations
+    #check atom.parent().altloc for whether any atoms have alt positions
     #only run this if conformer.altloc != ''
-    #atom.id_str() looks like 'pdb=" C1'B  G B  -3 "' for a B alternate
-    #this format may change with mmCIF
     for atom in [residue_1_c1p_outbound_atom, residue_2_p_atom]:
       if atom is None: continue
-      altloc = atom.id_str()[9:10]
-      if altloc != " ":
+      altloc = atom.parent().altloc
+      if altloc != "":
         return altloc
     for atom in residue_1_deoxy_ribo_atom_dict.values():
       if atom is None: continue
-      altloc = atom.id_str()[9:10]
-      if altloc != " ":
+      altloc = atom.parent().altloc
+      if altloc != "":
         return altloc
     return ""