NAME:
    boxm
 PURPOSE: (one line)
    Find location of a maximum within a sub-array.
 DESCRIPTION:

 CATEGORY:
  CCD data processing
 CALLING SEQUENCE:
    boxm, image, xcen, ycen, deltax, deltay, xmax, ymax
 INPUTS:
    image      : Image array.
    xcen, ycen : Center of sub-array.
    deltax     : Half-width of sub-array.
    deltay     : Half-height of sub-array.

 OPTIONAL INPUT PARAMETERS:

 KEYWORD PARAMETERS:
    ABSMAX     : Flag, if set, looks for pixel with greatest absolute value.

 OUTPUTS:
    xmax, ymax : Coordinates, in image, of local maximum.

 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).
    1/26/94, MWB, Added ABS argument.
    12/13/95, MWB, added support for 3-d input arrays.
    98/09/21, MWB, added NOCHECK keyword to speed up execution.
    2002/07/12, MWB, changed function so that xcen,ycen,deltax,deltay
                  are changed to long.  xcen,ycen are rounded, deltax/y
                  are truncated.