[MatL] Use make_tuple for return value construct.

This is not working otherwise for either to old (c++11) STL implementation or boost library version.

MaterialLib/SolidModels/Ehlers-impl.h

@@ -739,13 +739,13 @@ SolidEhlers<DisplacementDim>::integrateStress(
     if (_damage_properties)
         sigma_final *= 1 - state.damage.value();
 
-    return {
-        {sigma_final,
-         std::unique_ptr<
-             typename MechanicsBase<DisplacementDim>::MaterialStateVariables>{
-             new StateVariables<DisplacementDim>{
-                 static_cast<StateVariables<DisplacementDim> const&>(state)}},
-         tangentStiffness}};
+    return {std::make_tuple(
+        sigma_final,
+        std::unique_ptr<
+            typename MechanicsBase<DisplacementDim>::MaterialStateVariables>{
+            new StateVariables<DisplacementDim>{
+                static_cast<StateVariables<DisplacementDim> const&>(state)}},
+        tangentStiffness)};
 }
 
 }  // namespace Ehlers

MaterialLib/SolidModels/LinearElasticIsotropic.h

@@ -108,13 +108,14 @@ public:
 
         KelvinVector sigma = sigma_prev + C * (eps - eps_prev);
 
-        return {{sigma,
-                 std::unique_ptr<typename MechanicsBase<
-                     DisplacementDim>::MaterialStateVariables>{
-                     new MaterialStateVariables{
-                         static_cast<MaterialStateVariables const&>(
-                             material_state_variables)}},
-                 C}};
+        return {
+            std::make_tuple(sigma,
+                            std::unique_ptr<typename MechanicsBase<
+                                DisplacementDim>::MaterialStateVariables>{
+                                new MaterialStateVariables{
+                                    static_cast<MaterialStateVariables const&>(
+                                        material_state_variables)}},
+                            C)};
     }
 
 private:

MaterialLib/SolidModels/Lubby2-impl.h

@@ -180,13 +180,15 @@ Lubby2<DisplacementDim>::integrateStress(
 
     // Hydrostatic part for the stress and the tangent.
     double const eps_i_trace = Invariants::trace(eps);
-    return {
-        {local_lubby2_properties.GM0 * sigd_j +
-             local_lubby2_properties.KM0 * eps_i_trace * Invariants::identity2,
-         std::unique_ptr<
-             typename MechanicsBase<DisplacementDim>::MaterialStateVariables>{
-             new MaterialStateVariables{state}},
-         C}};
+    KelvinVector const sigma =
+        local_lubby2_properties.GM0 * sigd_j +
+        local_lubby2_properties.KM0 * eps_i_trace * Invariants::identity2;
+    return {std::make_tuple(
+        sigma,
+        std::unique_ptr<
+            typename MechanicsBase<DisplacementDim>::MaterialStateVariables>{
+            new MaterialStateVariables{state}},
+        C)};
 }
 
 template <int DisplacementDim>