NAME:
  spexpand
 PURPOSE:   (one line only)
  Sparse expansion of an array
 DESCRIPTION:
  This routine expands an image by in integer factor in both dimensions.
  The original pixels from the input image are carried over to the LLHC
  of the nxn expanded area that maps back to the pixel.  The other pixels
  in the expanded image are set to 0.  This effectively converts the image
  into a 2-D shah (III) function.

  This routine does a 2x2 expansion (could be generalized if needed).

  The purpose of this routine is to support the reconstruction of
  undersampled dithered images.  (see XXX.pro)

 CATEGORY:
  Image Processing
 CALLING SEQUENCE:
  result=spexpand(image)
 INPUTS:
  image - Input image to be expanded.
 OPTIONAL INPUT PARAMETERS:
 KEYWORD INPUT PARAMETERS:
  ROOT2 - Flag, if set requests an expansion of the image by sqrt(2).
            The geometry of this necessitates a 45 degree rotation of the
            image.  The default is a 2x2 expansion.
 OUTPUTS:
  return value is the expanded image
 KEYWORD OUTPUT PARAMETERS:
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
 PROCEDURE:
 MODIFICATION HISTORY:
  2016/09/06, Written by Marc W. Buie, Southwest Research Institute
  2017/01/12, MWB, added ROOT2 keyword option