NAME:
    centrod
 PURPOSE: (one line)
    Compute center of mass of an object aperture.
 DESCRIPTION:

 CATEGORY:
  CCD data processing
 CALLING SEQUENCE:
    centrod, image, xcen, ycen, radius, inradius, outradius, skyback, $
             xbar, ybar, counts
 INPUTS:
    image          : CCD image array.
    xcen, ycen     : Center of window.
    radius         : Radius of window.
    inradius       : Inner radius of sky annulus.
    outradius      : Outer radius of sky annulus.
    skyback        : Sky background in counts per pixel.
 OPTIONAL INPUT PARAMETERS:

 KEYWORD PARAMETERS:
    ADDNEG         : Flag, if set, the position is computed using both
                       the moments of counts above and below the background.
                       Default is to compute the position only using those
                       image values greater than the sky background.

 OUTPUTS:
    xbar, ybar     : Position of center of mass.
    counts         : Mass of object in counts.
 COMMON BLOCKS:

 SIDE EFFECTS:

 RESTRICTIONS:

 PROCEDURE:

 MODIFICATION HISTORY:
    Ported by Doug Loucks, Lowell Observatory, 1992 Oct, from the C-
    language version written by Marc Buie.
    4/1/93, DWL, Added argument validation (badpar).
    99/08/31, MWB, added ADDNEG keyword.