Add sulphur and aluminium aliases.

Fix small bug in internal/pdbstructure.py.

wrappers/python/simtk/openmm/app/element.py

@@ -203,3 +203,8 @@ ununtrium =      Element(113, "ununtrium",      "Uut", 284*daltons)
 ununquadium =    Element(114, "ununquadium",    "Uuq", 289*daltons)
 ununpentium =    Element(115, "ununpentium",    "Uup", 288*daltons)
 ununhexium =     Element(116, "ununhexium",     "Uuh", 292*daltons)
+
+# Aliases to recognize common alternative spellings. Both the '==' and 'is'
+# relational operators will work with any chosen name
+sulphur = sulfur
+aluminium = aluminum

wrappers/python/simtk/openmm/app/internal/pdbstructure.py

@@ -436,7 +436,7 @@ class Chain(object):
         self._finalize()
 
     def get_residue(self, residue_number, insertion_code=' '):
-        return residues_by_num_icode[str(residue_number) + insertion_code]
+        return self.residues_by_num_icode[str(residue_number) + insertion_code]
 
     def __contains__(self, residue_number):
         return self.residues_by_number.__contains__(residue_number)
@@ -916,7 +916,6 @@ if __name__=='__main__':
     import doctest, sys
     doctest.testmod(sys.modules[__name__])
 
-    import sys
     import os
     import gzip
     import re