fouriercoeffs_tf

  FOURIERCOEFFS_TF  Fourier Transform at a particular frequency.
    ft(response, tf, SAMPLERATE) is the product of response with
    exp( 2*pi*i*tf/SAMPLERATE ). If tf is zero it returns the mean.
    If response is two-dimensional, ft operates on the columns and
    returns a row vector.
    tf is expressed in whatever units SAMPLERATE is expressed in 
    (I usually use Hz). Only the data within an integer number of periods 
    are considered.
    From Sooyoung Chung, from Matteo Carandini,modified slightly by
    Steve Van Hooser