cut DownSample>1 K-modes and only save the high freq disp/delta

reassemble them and combine with a nearest neighbour interpolation
from high modes in convert.py.
only save the seedtable that may be useful.(Nsample **2 points)
do not open the file if it is not needed.
use FFTW_MPI_TRANSPOSE_IN for the inverse transform.

will save us unnecessary communication time.