adds drop_voxels_near_border as df_filter
adds drop_voxels_near_border as df_filter

adds a new df_filter, drop_voxels_near_border, which allows user to drop
all voxels whose pRF center lie closer than 1 pRF sigma to the
border. Currently, no way to set border values from command-line / with
construct_df_filter function (though we should no longer be changing
that value, so that's probably fine)

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