vlt.math.fouriercoeffs_tf2

  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). 
    From Sooyoung Chung, from Matteo Carandini,modified slightly by
    Steve Van Hooser