[utils] VerticalSliceExtraction tool extension to support generating (incomplete) 2nd order elements

Adjusts VerticalSliceExtraction tool such that (incomplete) second order elements can be created via the GMSH call.

@dominik-kern As far as I can tell, the resulting GMSH output (available via the -t argument is exactly what you need. Unfortunately, the OGS GMSH interface does at this point not support higher order elements. So at the moment the tool fails after generating the (higher order) GMSH mesh and is therefore only of limited use.

Maybe @TomFischer can help with this as he was the one originally writing the GMSH interface?