Autodetect the large digital output from the encoder in the first frame of the session

[arielleleon]

The encoder displays a high value at the start of the stimulus. There is another issues that will filter out this frame by re-sizing the y-axes of the encoder plot. While the high event can be ignored, it is also something to track. We should auto-detect when the first encoder value is high.

Example session where this occurs: http://mouse-seeks/qc/ophys/session/1287539392