Add a basic DCD file test. There's no DCD file _reader_, so without MDTraj we

can't validate the that this object does anything more than simply write without
exceptions (i.e., we can't tell if the DCD file is actually _valid_)

wrappers/python/tests/TestDcdFile.py

+import unittest
+import tempfile
+import numpy as np
+from simtk.openmm import app
+import simtk.openmm as mm
+from simtk import unit
+from random import random
+import os
+
+class TestDCDFile(unittest.TestCase):
+    def test_dcd(self):
+        """ Test the DCD file """
+        pdbfile = app.PDBFile('systems/alanine-dipeptide-implicit.pdb')
+        natom = len(list(pdbfile.topology.atoms()))
+        dcd = app.DCDFile(open('test.dcd', 'w'), pdbfile.topology, 0.001)
+        for i in range(5):
+            dcd.writeModel([mm.Vec3(random(), random(), random()) for j in range(natom)]*unit.angstroms)
+        dcd._file.close()
+        os.remove('test.dcd')
+
+if __name__ == '__main__':
+    unittest.main()