ri_id	IN:	Raster image identifier returned by GRcreate or GRselect
	filename	IN:	Name of the external file
	offset	IN:	Offset in bytes from the beginning of the external file to where the data will be written

	Purpose	Specifies that the raster image will be written to an external file.
	Return value	Returns SUCCEED (or 0) if successful and FAIL (or -1) otherwise.
	Description	GRsetexternalfile specifies that the raster image identified by the parameter ri_id will be written to the external file specified by the parameter filename at the offset specified by the parameter offset.
		Data can only be moved once for any given raster image, and it is the user's responsibility to make sure the external data file is kept with the "original" file.
		If the raster image already exists, its data will be moved to the external file . Space occupied by the data in the primary file will not be released. To release the space in the primary file use the hdfpack command-line utility. If the raster image does not exist, its data will be written to the external file during the subsequent calls to GRwritedata.
		See the reference manual entries for HXsetcreatedir and HXsetdir for more information on the options available for accessing external files.

	FORTRAN	integer function mgsxfil(ri_id, filename, offset)
		integer ri_id, offset
		character*(*) filename