Vertically summed slices are transposed (2decomp)

[BenMql]

In the master branch (2decomp library and pencils domain decomposition), vertically summed slices Zsummed are transposed compared to other slices in the XY plane, ZSlice. For Zsummed slices, the fast index in memory corresponds to x (and y is slow).

This does not happen with the WiP branch (fftw-mpi and slabs domain decomposition), where the output has a consistent memory layout: x is slowest, then y, then z is fastest.