major fixes/changes to sanDiegoFFT
major fixes/changes to sanDiegoFFT

Removal of 0Hz freq: The signal is substracted by the mean before
computing the FFT coefs to find the dominant frequencies. Otherwise,
the dominant frequency (p1, f1) will always be 0Hz, making these
features uninformative.

Fix spectral entropy: The calculation of the spectral entropy seemed
slightly wrong, using the magnitudes instead of the powers. There was
also some strange normalization applied (magDC = vmFFT[0] / max).
This is now fixed to correctly use the powers, and normalization is also
done correctly.

Fix p1, p33 (dominant powers): Again, these were using the magnitudes
instead of the powers as in the papers. A strange normalization is also
applied (dividing by max of the FFT coefs). This is now fixed to use the
powers. Instead of the strange normalization, use logscale to handle the
large values that p1, p33 tend to have.

Fix magnitude estimations (FFT0-FFT14): Magnitudes for 0Hz-14Hz (note:
this is different from paper's 1Hz-15Hz) were computed using Welch's
method, but the implementation had some mistakes. First, the running
window does not reach till the end of the signal, only until half of it.
Second, Welch's method uses overlapping windows. Another change in
the estimation is to use logscale instead of the strange normalization.

computationalEpidemiology/biobankAccelerometerAnalysis,aidendoherty/biobankAccelerometerAnalysis,computationalEpidemiology/biobankAccelerometerAnalysis,aidendoherty/biobankAccelerometerAnalysis