Let SumGridFunction use const references for BaseType functions.

dune/fufem/functions/sumgridfunction.hh

@@ -112,7 +112,7 @@ public:
      * \param coefficient of summand function
      * \param function summand (grid-)function to register
      */
-    virtual void registerFunction(double coefficient, BaseType& function)
+    virtual void registerFunction(double coefficient, const BaseType& function)
     {
         functions_.push_back(&function);
         coefficients_.push_back(coefficient);
@@ -121,7 +121,7 @@ public:
     ~SumGridFunction(){}
 
 private:
-    std::vector<BaseType*> functions_;
+    std::vector<const BaseType*> functions_;
     std::vector<double> coefficients_;
 
 };