severe overhaul of GLM df creation
severe overhaul of GLM df creation

this is a pretty big overhaul of how I create the GLM results
dataframe. There are two big categories of changes: I now explicitly
limit the content of the dataframe to one of two "modes", full or
summary (and this specifies which mgz files I load in and how they're
handled), and I do all the limiting (both of visual area and
eccentricity) and binning (just of eccentricity) while loading in the
mgz files, which makes things somewhat more efficient and results in a
much smaller dataframe being created

I also split up the mammoth create_GLM_result_df into several smaller
functions, also round the frequency space distance when creating the df,
and make it possible to save the dataframe during creation

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