UI-QT: Minor bugfix for for isosurfaces of some 3D systems.

If a system has no translations of the basis cell, it can still be 3D and the isosurfaces should be drawn. This was prevented by an incorrect usage of n_cell_step, which regulates the resolution of basis cells.

ui-cpp/src/SpinWidget.cpp

@@ -392,7 +392,7 @@ void SpinWidget::updateVectorFieldGeometry()
     //      get tetrahedra
     if (Geometry_Get_Dimensionality(state.get()) == 3)
     {
-        if (n_cells[0]/n_cell_step < 2 || n_cells[1]/n_cell_step < 2 || n_cells[2]/n_cell_step < 2)
+        if( n_cell_step > 1 && n_cells[0]/n_cell_step < 2 || n_cells[1]/n_cell_step < 2 || n_cells[2]/n_cell_step < 2 )
         {
             geometry = VFRendering::Geometry(positions, {}, {}, true);
         }