VisualizationMesh material numbering (#38)

* Changed material numbering in lsVisusalizationMesh to always have lowest material at 0.

include/lsWriteVisualizationMesh.hpp

@@ -603,7 +603,7 @@ template <class T, int D> class lsWriteVisualizationMesh {
            materialMeshes[materialMeshes.size() - 1 - i]->GetNumberOfCells();
            ++j) {
         materialNumberArray->InsertNextValue(
-            materialIds[materialMeshes.size() - 1 - i]);
+            materialIds[i]);
       }
       materialMeshes[materialMeshes.size() - 1 - i]->GetCellData()->SetScalars(
           materialNumberArray);