Changed Hex edge numbering

Changed MeshLib::Hex edge numbering to match VTK_HEXAHEDRON edge numbering.
(Our order was bottom-face-edges -> vertical edges -> top-face-edges, VTK uses bottom-face-edges -> top-face-edges -> vertical edges.)

As far as I can see this has no impact on any functionality.
It *might* be relevant for @TomFischer 's ```reviseElement()```-methods but if it is I don't see how.