Fix a bug with computation of VonMises Stress. Material law data is now

removed from DGelasticField which contains a pointer to a materialLaw.
Update benchmarks to include this change 

Geo/SVector3.h

@@ -27,7 +27,7 @@ class SVector3 {
   inline double x(void) const { return P.x(); }
   inline double y(void) const { return P.y(); }
   inline double z(void) const { return P.z(); }
-  inline double norm() { return sqrt(P[0] * P[0] + P[1] * P[1] + P[2] * P[2]); }
+  inline double norm() const { return sqrt(P[0] * P[0] + P[1] * P[1] + P[2] * P[2]); }
   inline double normSq() { return (P[0] * P[0] + P[1] * P[1] + P[2] * P[2]); }
   double normalize()
   {