Rename SFTF to FFT and add a few comments
Rename SFTF to FFT and add a few comments

joshblum/eeg-spectrogram,joshblum/eeg-toolkit,joshblum/eeg-toolkit,joshblum/eeg-toolkit,joshblum/eeg-spectrogram,joshblum/eeg-toolkit,joshblum/eeg-spectrogram,joshblum/eeg-toolkit,joshblum/eeg-spectrogram,joshblum/eeg-spectrogram