add third derivatives in Element

Geo/MElement.cpp

@@ -142,6 +142,13 @@ void MElement::getHessShapeFunctions(double u, double v, double w, double s[][3]
   else Msg::Error("Function space not implemented for this type of element");
 }
 
+void MElement::getThirdDerivativeShapeFunctions(double u, double v, double w, double s[][3][3][3],
+                                     int o){
+  const polynomialBasis* fs = getFunctionSpace(o);
+  if(fs) fs->dddf(u, v, w, s);
+  else Msg::Error("Function space not implemented for this type of element");
+};
+
 SPoint3 MElement::barycenter()
 {
   SPoint3 p(0., 0., 0.);

Geo/MElement.h

@@ -235,6 +235,8 @@ class MElement
                                      int order=-1);
   virtual void getHessShapeFunctions(double u, double v, double w, double s[][3][3],
                                      int order=-1);
+  virtual void getThirdDerivativeShapeFunctions(double u, double v, double w, double s[][3][3][3],
+                                     int order=-1);
   // return the Jacobian of the element evaluated at point (u,v,w) in
   // parametric coordinates
   double getJacobian(const fullMatrix<double> &gsf, double jac[3][3]);