Let SumFunction use pointer to const functions.

The BaseType functions are VirtualFunctions exhibiting
const methods only anyway, so it should accept const
BaseType functions directly.
Otherwise explicit and unneccessary const_cast is needed
when SumFunctions are used.

dune/fufem/functions/sumfunction.hh

@@ -33,7 +33,7 @@ public:
     /** \brief Constructor
      *
      */
-    SumFunction(std::vector<BaseType*>& functions, std::vector<double>& coefficients):
+    SumFunction(std::vector<const BaseType*>& functions, std::vector<double>& coefficients):
         functions_(functions),
         coefficients_(coefficients)
     {
@@ -70,7 +70,7 @@ public:
      * \param coefficient for summand function
      * \param function summand 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);
@@ -79,7 +79,7 @@ public:
     ~SumFunction(){}
 
 private:
-    std::vector<BaseType*> functions_;
+    std::vector<const BaseType*> functions_;
     std::vector<double> coefficients_;
 
 };