add nfft in mhd which is then used in gdat_plot for spectrograms time...

add nfft in mhd which is then used in gdat_plot for spectrograms time resolution, changed default to 512 from 1024

git-svn-id: https://spcsvn.epfl.ch/repos/TCV/gdat/trunk@8824 d63d8f72-b253-0410-a779-e742ad2e26cf