make the name() method of problems non-static again

this allows to easily specify the problem name at runtime.

examples/problems/diffusionproblem.hh

@@ -189,7 +189,7 @@ public:
     /*!
      * \copydoc VcfvProblem::name
      */
-    static std::string name()
+    std::string name() const
     { return std::string("diffusion_") + Model::name(); }
 
     //! \}