In real space processors are organized in a 2D pattern.

Each processor owns data from a sub-set of Z-planes and a sub-set of Y-planes.
In reciprocal space each processor owns Z-columns that belong to a sub set of
X-values. This allows to split the processors in two sets for communication
in the YZ and XY planes.
In alternative, if the situation allows for it, a task group paralelization is used
(with ntg=nyfft) where complete XY planes of ntg wavefunctions are collected and Fourier
trasnformed in G space by different task-groups. This is preferable to the Z-proc + Y-proc
paralleization if task group can be used because a smaller number of larger ammounts of 
data are transferred. Hence three types of fft are implemented: 
 
  !
  !! ... isgn = +-1 : parallel 3d fft for rho and for the potential
  !
  !! ... isgn = +-2 : parallel 3d fft for wavefunctions
  !
  !! ... isgn = +-3 : parallel 3d fft for wavefunctions with task group
  !
  !! ... isgn = +   : G-space...