NAME:
  gridwt
 PURPOSE:
  Compute circle overlap weights in a circle within an array.
 DESCRIPTION:
 CATEGORY:
  CCD data processing
 CALLING SEQUENCE:
  gridwt,xc,yc,radius,xsize,ysize,wtarr
 INPUTS:
  xc     - X center of circle.
  yc     - Y center of circle.
  radius - Radius of the circle in pixels.
  xsize  - X size of array
  ysize  - Y size of array
 OPTIONAL INPUT PARAMETERS:
 KEYWORD INPUT PARAMETERS:
 OUTPUTS:
  wtarr  - Output array filled with overlap area of each pixel with given
            circle.  Values range from 0 to 1.
 KEYWORD OUTPUT PARAMETERS:
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
 PROCEDURE:
 MODIFICATION HISTORY:
  97/11/12 - Written by Marc W. Buie