Added Python test for Topology getters.

wrappers/python/tests/TestTopology.py

+import sys
+import unittest
+from simtk.openmm.app import *
+from simtk.openmm import *
+from simtk.unit import *
+import simtk.openmm.app.element as elem
+if sys.version_info >= (3, 0):
+    from io import StringIO
+else:
+    from cStringIO import StringIO
+
+
+class TestTopology(unittest.TestCase):
+    """Test the Topology object"""
+
+
+    def check_pdbfile(self, pdbfilename, natoms, nres, nchains):
+        """Check that a PDB file has the specified number of atoms, residues, and chains."""
+        pdb = PDBFile(pdbfilename)
+        top = pdb.topology
+        self.assertEqual(pdb.topology.getNumAtoms(), natoms)
+        self.assertEqual(pdb.topology.getNumResidues(), nres)
+        self.assertEqual(pdb.topology.getNumChains(), nchains)
+
+    def test_getters(self):
+        """Test getters for number of atoms, residues, chains."""
+        check_pdbfile('systems/1T2Y.pdb', 271, 25, 1)
+
+if __name__ == '__main__':
+    unittest.main()