make helper function const

[[Imported from SVN: r11297]]

dune/elasticity/assemblers/neohookeoperatorassembler.hh

@@ -192,7 +192,7 @@ private:
      *  Note that it is assumed the defGrad is the deformation gradient and not the displacement gradient   
      */
     Dune::FieldMatrix<ctype,dim,dim> hessDefDet(const Dune::FieldMatrix<ctype,dim,dim>& defGrad,
-                            const Dune::FieldVector<ctype,dim>& testGrad, const Dune::FieldVector<ctype,dim>& ansatzGrad)
+                            const Dune::FieldVector<ctype,dim>& testGrad, const Dune::FieldVector<ctype,dim>& ansatzGrad) const
     {
         Dune::FieldMatrix<ctype, dim, dim> res(0);