Remove the stray light from the images

* remove-stray-light.py: This uses an ad hoc function to fit the
  observed stray light, which is (1) A term that is constant with
  wavelength, but linear with order, plus (2) a term that is constant
  with order and x*exp(-x) with wavelength.  This is tailored for the
  p84 exposure.  We ought to check that it doesn't change for the other
  exposures.
Allow order removal from CR-cleaned images

* extract-orders.py: No longer assume that there is only one HDU in the
  image file, since this is not true if it has been CR-cleaned.