Made changes to print peak I and peak pol, and to calculate the average fractional polarization based on pol and I values in the same pixels.  Those numbers are written to pol_frac_avg.txt.  Also, about to make big changes to array sizes based on read-in imsize, rather than hard-coded 512 and 256.
Makes a plot of large-vs-small-scale-B-field angle vs. average fractional polarization.
Now reads in and prints average polarization percentage into a new column in the table.
Instead of assuming imsize is 512x512, it now reads the size and sets the center to the length and width values /2.