Initialize using the correct type

dune/elasticity/common/elasticityhelpers.hh

@@ -18,7 +18,7 @@ namespace Dune {
          */
         template <int dim, class field_type=double>
         void computeNonlinearStrain(const Dune::FieldMatrix<field_type, dim, dim>& grad, SymmetricTensor<dim,field_type>& strain) {
-            strain = 0;
+            strain = field_type(0);
             for (int i=0; i<dim ; ++i) {
                 strain(i,i) +=grad[i][i];