[MatL] Solids; Move up default impl of push back.

MaterialLib/SolidModels/LinearElasticIsotropic.h

@@ -62,7 +62,6 @@ public:
     struct MaterialStateVariables
         : public MechanicsBase<DisplacementDim>::MaterialStateVariables
     {
-        void pushBackState() override {}
     };
 
     std::unique_ptr<

MaterialLib/SolidModels/LinearElasticOrthotropic.h

@@ -126,7 +126,6 @@ public:
     struct MaterialStateVariables
         : public MechanicsBase<DisplacementDim>::MaterialStateVariables
     {
-        void pushBackState() override {}
     };
 
     std::unique_ptr<

MaterialLib/SolidModels/MechanicsBase.h

@@ -57,7 +57,7 @@ struct MechanicsBase
     struct MaterialStateVariables
     {
         virtual ~MaterialStateVariables() = default;
-        virtual void pushBackState() = 0;
+        virtual void pushBackState(){};
     };
 
     /// Polymorphic creator for MaterialStateVariables objects specific for a