Revert "[operators] do not be unconditionally linear"

This reverts commit 61cf3181.

dune/gdt/operators/advection-dg.hh

@@ -129,11 +129,6 @@ public:
 
   AdvectionDgOperator(ThisType&& source) = default;
 
-  bool linear() const override final
-  {
-    return numerical_flux_->linear();
-  }
-
   using BaseType::append;
 
   /// \name Non-periodic boundary treatment

dune/gdt/operators/advection-fv.hh

@@ -88,11 +88,6 @@ public:
     , periodicity_exception_(std::move(source.periodicity_exception_))
   {}
 
-  bool linear() const override final
-  {
-    return numerical_flux_->linear();
-  }
-
   using BaseType::append;
 
   /// \name Non-periodic boundary treatment

dune/gdt/operators/localizable-operator.hh

@@ -280,11 +280,17 @@ public:
     , assembly_grid_view_(assembly_grid_view)
     , source_space_(source_space)
     , range_space_(range_space)
+    , linear_(true)
     , use_tbb_(use_tbb)
   {}
 
   LocalizableOperator(ThisType&& source) = default;
 
+  bool linear() const override final
+  {
+    return linear_;
+  }
+
   const SourceSpaceType& source_space() const override final
   {
     return source_space_;