ENH: adding const access to reactingMixture list

src/thermophysicalModels/reactionThermo/mixtures/reactingMixture/reactingMixture.H

@@ -2,7 +2,7 @@
   =========                 |
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     |
-    \\  /    A nd           | Copyright (C) 2011 OpenFOAM Foundation
+    \\  /    A nd           | Copyright (C) 2011-2013 OpenFOAM Foundation
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
 License
@@ -96,6 +96,11 @@ public:
         {
             return PtrList<Reaction<ThermoType> >::operator[](i);
         }
+
+        const Reaction<ThermoType>& operator [] (const label i) const
+        {
+            return PtrList<Reaction<ThermoType> >::operator[](i);
+        }
 };