Let ComposedFunction have const references of subfunctions.

dune/fufem/functions/composedfunction.hh

@@ -31,7 +31,7 @@ public:
      *  \param f_ outer function for composition
      *  \param g_ inner function for composition
      */
-    ComposedFunction(OuterFunctionType& f_, InnerFunctionType& g_) :
+    ComposedFunction(const OuterFunctionType& f_, const InnerFunctionType& g_) :
         f(f_),
         g(g_)
     {}
@@ -52,8 +52,8 @@ public:
     ~ComposedFunction(){}
 
 private:
-    OuterFunctionType& f;
-    InnerFunctionType& g;
+    const OuterFunctionType& f;
+    const InnerFunctionType& g;
 
 };