NAME:
  ss_group
 PURPOSE:   (one line only)
  Simple Spectra: Average a group of spectra, correct to mean airmass
 DESCRIPTION:
 CATEGORY:
  Spectroscopy
 CALLING SEQUENCE:
  ss_group,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 averaged spectrum
 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