Add __reduce__ method to rmgpy.rmg.pdep.RDepReaction (for pickling).

Hopefully closes #114

rmgpy/rmg/pdep.py

@@ -60,7 +60,13 @@ class PDepReaction(rmgpy.reaction.Reaction):
     def __init__(self, index=-1, label='', reactants=None, products=None, network=None, kinetics=None, reversible=True, transitionState=None, duplicate=False, degeneracy=1, pairs=None):
         rmgpy.reaction.Reaction.__init__(self, index, label, reactants, products, kinetics, reversible, transitionState, duplicate, degeneracy, pairs)
         self.network = network
-
+
+    def __reduce__(self):
+        """
+        A helper function used when pickling an object.
+        """
+        return (PDepReaction, (self.index, self.label, self.reactants, self.products, self.network, self.kinetics, self.reversible, self.transitionState, self.duplicate, self.degeneracy, self.pairs))
+
     def getSource(self):
         """
         Get the source of this PDepReaction