Make default constructors default

dune/elasticity/materials/mooneyrivlinmaterial.hh

@@ -63,12 +63,7 @@ private:
     using MonRivHessian = MooneyRivlinOperatorAssembler<GridType, Lfe, Lfe>;
 
 public:
-    MooneyRivlinMaterial(int k=3) :
-        E_(100), nu_(0.3), k_(k)
-    {
-        setupCoefficients();
-        constructAssemblers();
-    }
+    MooneyRivlinMaterial() = default;
 
     template <class BasisT>
     MooneyRivlinMaterial(BasisT&& basis, field_type E, field_type nu, int k=3) :

dune/elasticity/materials/neohookeanmaterial.hh

@@ -64,10 +64,7 @@ private:
     using NeoHessian = NeoHookeOperatorAssembler<GridType, Lfe, Lfe>;
 
 public:
-    NeoHookeanMaterial() :
-        lambda_(1.0),
-        mu_(0.3)
-    {}
+    NeoHookeanMaterial() = default;
 
     template <class BasisT>
     NeoHookeanMaterial(BasisT&& basis, ReturnType E, ReturnType nu) :