fouriercoeffs_tf_spikes

  FOURIERCOEFFS_TF_SPIKES  Fourier Transform of a spike train at a particular frequency.

   F = FOURIERCOEFFS_TF_SPIKES(SPIKETIMES, TF, DURATION)
      This function returns the normalized fourier coefficient of 
      a spike train that is defined by an array of SPIKETIMES with a 
      duration DURATION.

      The function calculates (2/DURATION) * exp(-2*pi*sqrt(-1)*tf).
      If tf is zero it returns the number of spikes times divided by the duration.
      Modified from Sooyoung Chung, from Matteo Carandini, by Steve Van Hooser