[cleanup] Remove const on return type in return by value functions

dumux/discretization/staggered/elementfacevariables.hh

@@ -169,7 +169,7 @@ public:
 
 private:
 
-    const int getLocalIdx_(const int scvfIdx) const
+    int getLocalIdx_(const int scvfIdx) const
     {
         auto it = std::find(faceVarIndices_.begin(), faceVarIndices_.end(), scvfIdx);
         assert(it != faceVarIndices_.end() && "Could not find the current face variables for scvfIdx!");

dumux/discretization/staggered/elementvolumevariables.hh

@@ -253,7 +253,7 @@ public:
 private:
     const GridVolumeVariables* gridVolVarsPtr_;
 
-    const int getLocalIdx_(const int volVarIdx) const
+    int getLocalIdx_(const int volVarIdx) const
     {
         auto it = std::find(volVarIndices_.begin(), volVarIndices_.end(), volVarIdx);
         assert(it != volVarIndices_.end() && "Could not find the current volume variables for volVarIdx!");

dumux/io/vtkoutputmodule.hh

@@ -241,9 +241,9 @@ protected:
     const GridVariables& gridVariables() const { return gridVariables_; }
     const SolutionVector& sol() const { return sol_; }
 
-    const bool verbose() const { return verbose_; }
+    bool verbose() const { return verbose_; }
     const std::string& name() const { return name_; }
-    const Dune::VTK::DataMode dataMode() const { return dm_; }
+    Dune::VTK::DataMode dataMode() const { return dm_; }
 
     Dune::VTKWriter<GridView>& writer() { return *writer_; }
     Dune::VTKSequenceWriter<GridView>& sequenceWriter() { return sequenceWriter_; }