[localizable-flux-function] add is_affine() method to interface

dune/xt/functions/affine.hh

@@ -501,6 +501,11 @@ public:
     BaseType::template helper<>::jacobian_col(col, A_, ret);
   }
 
+  virtual bool is_affine() const override
+  {
+    return true;
+  }
+
 private:
   using BaseType::A_;
   using BaseType::b_;

dune/xt/functions/interfaces/localizable-flux-function.hh

@@ -68,6 +68,11 @@ public:
 
   virtual std::unique_ptr<LocalfunctionType> local_function(const EntityType& /*entity*/) const = 0;
 
+  virtual bool is_affine() const
+  {
+    return false;
+  }
+
   /**
    * \name ´´These methods should be implemented in order to identify the function.''
    * \{