NAME:
  ss_extin
 PURPOSE:   (one line only)
  Simple Spectra - detemine differential extinction
 DESCRIPTION:
 CATEGORY:
  Spectroscopy
 CALLING SEQUENCE:
  ss_extin,root,numlist,outfile
 INPUTS:
  root    - String, root of file name
  numlist - String, list of spectrum numbers to load.  For a full description
               of the syntax allowed, see RANGEPAR.PRO.  A quick example:
                 '10-15,20-25' would load numbers 10 through 15, inclusive
                 and 20 through 25, inclusive.

    The file names constructed will be root.NNNN.spec and is expected to
       be a FITS file with the first column being wavelength and the second
       being the spectrum.
  outfile - String, name of output extinction coefficient file.  The
               file is in FITS format and writes three columns, the first
               is wavelength (same vector) as from original data.  The
               second column is extinction in magnitudes per airmass
               relative to the reference wavelength.  The extinction at
               the reference wavelength is defined to be zero.  A positive
               value has a higher extinction than the reference and a
               negative value indicates lower extinction than the reference
               wavelength.  The third column is the uncertainty on the
               extinction.  Note that NaN is used to flag wavelengths
               that have no valid data.

 OPTIONAL INPUT PARAMETERS:
 KEYWORD INPUT PARAMETERS:
  REFWAVE - reference wavelength in same units as wavelength vector in
               the spectrum file.  Default is 0.85 nm.
 OUTPUTS:
 KEYWORD OUTPUT PARAMETERS:
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
 PROCEDURE:
 MODIFICATION HISTORY:
  Written by Marc W. Buie, Lowell Observatory, 2004/06/14