bugfixes for getting bootstrap_num working
bugfixes for getting bootstrap_num working

 - we raise an exception if len(bootstrap_num) > 1, because I haven't
   fully thought through how to handle that yet, and I'm only planning
   on doing one bootstrap at a time

 - we filter the results_df by bootstrap_num when combining it with
   performance_d

 - we add bootstrap_num as a column to results, loss and model_history
   df in the same way we do test_subset

billbrod/spatial-frequency-preferences,billbrod/spatial-frequency-preferences,billbrod/spatial-frequency-preferences