NAME:
  mkxyarr
 PURPOSE:   (one line only)
  Create two arrays that contain the x,y positions of the elements in an array
 DESCRIPTION:
 CATEGORY:
  Miscellaneous
 CALLING SEQUENCE:
  mkxyarr,xsize,ysize,xarr,yarr
 INPUTS:
  xsize - x-dimension (width) of the array
  ysize - y-dimension (height) of the array
 OPTIONAL INPUT PARAMETERS:
 KEYWORD INPUT PARAMETERS:
  X0 - location of the X origin in the image, default=0
  Y0 - location of the Y origin in th eimage, default=0
  SCALE - scale factor of image pixel to your desired x,y scale,
             default=1.0
  Note: if all three are defaulted, the return arrays will be integers,
    otherwise you get back two floating point arrays.
 OUTPUTS:
  xarr  - Array of dimension [xsize,ysize] where each value is the x coordinate
             of the element.  The values range from 0 to xsize-1.
  yarr  - Array of dimension [xsize,ysize] where each value is the y coordinate
             of the element.  The values range from 0 to ysize-1.
 KEYWORD OUTPUT PARAMETERS:
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
 PROCEDURE:
 MODIFICATION HISTORY:
  2016/09/23, Written by Marc W. Buie, Southwest Research Institute, originally
                 as posarr.pro
  2019/12/20, renamed to mkxyarr with some style modifications