Lots of changes to LL arc fitting and analysis

New program:

* read-shapes-LL/arc_brightness.py
   - use masks to isolate the shell and bg
   - make plots

Modified programs:

* read-shapes-LL/extract-image.py
   - correct window around object

* read-shapes-LL/fit-circle-shell.py
   - write DS9 region file with the circle fits