bugfix: use the fd Hessian matrix for error norm computation

[[Imported from SVN: r1717]]

rod3d.cc

@@ -183,7 +183,7 @@ int main (int argc, char *argv[]) try
     MatrixIndexSet indices(exactSolution.size(), exactSolution.size());
     rodAssembler.getNeighborsPerVertex(indices);
     indices.exportIdx(hessian);
-    rodAssembler.assembleMatrix(exactSolution, hessian);
+    rodAssembler.assembleMatrixFD(exactSolution, hessian);
 
 
     double error = std::numeric_limits<double>::max();