DisorderedAtom returns children sorted by altloc (#3232)

* Added method to return sorted list of children of DisorderedAtom

* Tweaked test file to test for order of disordered children.

Bio/PDB/Atom.py

@@ -482,6 +482,13 @@ class DisorderedAtom(DisorderedEntityWrapper):
         """Return disordered atom identifier."""
         return "<Disordered Atom %s>" % self.get_id()
 
+    def disordered_get_list(self):
+        """Return list of atom instances.
+
+        Sorts children by altloc (empty, then alphabetical).
+        """
+        return sorted(self.child_dict.values(), key=lambda a: ord(a.altloc))
+
     def disordered_add(self, atom):
         """Add a disordered atom."""
         # Add atom to dict, use altloc as key

Tests/PDB/disordered.pdb

@@ -21,7 +21,7 @@ ATOM    218  CB  ARG A  27      56.899  20.533  28.660  1.00 16.52           C
 ATOM    219  CG  ARG A  27      57.901  20.081  27.621  1.00 19.45           C
 ATOM    220  CD  ARG A  27      58.541  21.359  27.019  1.00 22.58           C
 ATOM    221  NE  ARG A  27      59.504  20.850  26.023  1.00 26.89           N
-ATOM    222  CZ AARG A  27      59.081  20.674  24.762  0.50 26.69           C
+ATOM    222  CZ  ARG A  27      59.081  20.674  24.762  0.50 26.69           C
 ATOM    223  CZ BARG A  27      60.798  20.732  26.326  0.50 26.95           C
 ATOM    224  NH1AARG A  27      57.848  21.002  24.386  0.50 27.16           N
 ATOM    225  NH1BARG A  27      61.262  21.064  27.522  0.50 27.39           N