Add beta/theta ratio as control metric for evaluating algorithm performance

[jdpigeon]

As a sanity check, we should include a beta/theta ratio metric in our ML work. This should be relatively straightforward to do with the MNE library.

Details on the FFT and band means used in our app:

• FFT input length, FFT length, and sampling rate: 256
• Epoch length: 256 samples (1s)
• Epoch interval: 250 ms (4times/s)
• FFT buffer smoothed over last 4 epochs
• Theta range: 4-8hz
• Beta range: 13-30hz