First attempt at removing order overlap

* remove-overlap.py:  This version does not work very well.  The problem
  seems to be that I am using too large a section of the slit to
  calculate the average brightness, and the brightness is not uniform.
  I will nextr try it using a smaller section.
Improved version of overlap removal

* remove-overlap.py: Just use three pixels close to the edge.  Also,
  fiddle the y-positions of the orders a little bit.