Averaged Autopower Spectrum

This function computes an averaged autopower from given time domain data. The
program accepts different windows (hann, flat, and rect), different levels
of overlap, and different correction factors. The program outputs the
averaged autopower (Gxx),all of the linear spectra used (Gx), and an
frequency vector (Fx).
[Gxx,Gx,Fx] = avgGxx(wind,PercOverlap,CF,numAvgs,Fs,N,timeData)

Example:
[Gx,Gxx,Fx] = fftPlotData('hann',50,'ACF',10,3200,3200,timeData)
semilogy(Fx,Gxx)