couple small changes
couple small changes

couple small changes to first_level_analysis to clean it up:

 - splits _arrange_mgzs_into_dict into two functions:
   _arrange_mgzs_into_dict and _bin_mgzs_dict, so that the binning (both
   eccentricity and hemisphere) is done in a separate function.

 - since we check that the keys of benson_template_names are ['varea',
   'angle', 'eccen'], we use those keys explicitly, instead of referring
   back to the keys of that dictionary. makes it a little clearer

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