Convert feedback calibration fitting to DataFrame

 - Convert feedback calibration fitting to use `pandas.DataFrame`.
 - Separate feedback calibration fitting from updating of `calibration`
   instance.
Add section headings to guide process...

 - Add widget to select previously saved experiments.
  * This will need to be modified, since the path is currently
    hard-coded.
Add draft of pandas HV feed-back calibration
Add functions to fit/plot HV attenuation...

Add functions to fit and plot control board high-voltage attenuation
model.