[utils] adjusting vertitical slice tool to construct meshes from bottom to top

Small adjustment so the VerticalSlice-tool now builds meshes from bottom to top instead the other way around. This also fixes a (so far irrelevant) issue that used to occur with outcroppings towards the surface. Basically 2D slices are now constructed the same way that OGS also constructs 3D meshes.

Note: I had problems with GMSH refusing to mesh the geometries created with the changes in this MR. This appears to be fixed with the current GMSH version. It might make sense to set a minimum requirement for the GMSH-version used? Specifically, the issue occured with GMSH 4.5.6 and is fixed in GMSH 4.9.5. (pinging @bilke for this)