Replace input parameter references by pointer

[[Imported from SVN: r12668]]

dune/elasticity/materials/material.hh

@@ -41,15 +41,14 @@ class Material
             basis_ = &basis;
         }
 
-
         //! Evaluate the strain energy
-        virtual ReturnType energy(const GridFunction& displace) = 0;
+        virtual ReturnType energy(const GridFunction* displace) = 0;
 
         //! Return the local assembler of the first derivative of the strain energy at some displacement
-        virtual LocalLinearization& firstDerivative(const GridFunction& displace) = 0;
+        virtual LocalLinearization& firstDerivative(GridFunction* displace) = 0;
 
         //! Return the local assembler of the second derivative of the strain energy at some displacement
-        virtual LocalHessian& secondDerivative(const GridFunction& displace) = 0;
+        virtual LocalHessian& secondDerivative(GridFunction* displace) = 0;
 
         //! Return the global basis
         const Basis& basis() {return *basis_;}