LBM CodeGen and Field Layouts

The current code generation for LBM (the one that generates a lattice model) hard codedly assumes a field layout of fzyx, which is different from the default for fields in walberla (zyxf). The reason is (partially) that codegen most likely will use vectorization which only makes sense for the fzyx layout. For that reason, the following should be done:

• let the user specify upon generation of the lattice model which layout should be used
• assert internally that the zyxf layout is not used in combination with any vectorization since it will break the LBM kernel, leading to completely wrong results