Account for duration in signal-to-noise ratio calculation

[emthompson-usgs]

It is important that the SNR be normalized such that the result is not a function of the difference in duration between the signal and noise windows. Currently, we do not normalize by duration for the signal-to-noise ratio. We are currently taking the ratio of the Fourier transform of the signal and noise windows, and this is sensitive to the duration of the windows. I think that we should be using the power spectrum for the SNR but I need to do some testing to be sure.