three types of calls are possibles :  'Rho', 'Wave', 'tgWave'

   In order to enable an fft-type for a given grid the corresponding clock_labels must be set.
   One gives a name to desc%rho_clock_lable for 'Rho' type fft and a name to
   desc%wave_clock_lable for 'Wave' and 'tgWave' types. Whether tg is
   possible depends of the already defined value of desc%have_task_groups variable (mispell to be corrected soon).

   definining
      dffts%rho_clock_label='ffts', dffts%wave_clock_label='fftw',
      dfftp%rho_clock_label='fft', dfftt%rho_clock_label='fftc' and
      dfftt%wave_clock_label='fftcw'
   and changing
      'Dense'->'Rho', 'Smooth'->'Rho', 'Custom'->'Rho', 'CustomWave'->'Wave'
   the same clock names and the same overall behavior as with the old interface is obtained.