NAME:
  skyim
 PURPOSE:   (one line only)
  Calculate a smooth sky image from 2-d polynomial fit coefficients
 DESCRIPTION:
 CATEGORY:
  CCD data processing
 CALLING SEQUENCE:
  skyim,dimen,coeff,skyimage
 INPUTS:
  dimen   - Size of array to compute [nx,ny]
  coeff   - Coefficients of fit
  xorder  - order of fit to X direction (default=0, constant)
  yorder  - order of fit to Y direction (default=0, constant)
              Length of coeff must be (xorder+1)*(yorder+1)
 OPTIONAL INPUT PARAMETERS:
 KEYWORD INPUT PARAMETERS:
 OUTPUTS:
  skyimage - Smooth image of sky in image.
 KEYWORD OUTPUT PARAMETERS:
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
 PROCEDURE:
 MODIFICATION HISTORY:
  Written by Marc W. Buie, Lowell Observatory, 2004/07/06
  2010/11/30, MWB, fixed problem with constant sky value option