filename	IN:	Name of the file
	ref	IN:	Reference number for the next call to DF24getimage

	Purpose	Specifies the reference number of the next image to be read when DF24getimage is next called.
	Return value	Returns SUCCEED (or 0) if successful and FAIL (or -1) otherwise.
	Description	DF24readref is commonly used in conjunction with DFANlablist, which returns a list of labels for a given tag together with their reference numbers. It provides a means of non-sequentially accessing 24-bit raster images in a file.
		There is no guarantee that reference numbers appear in sequence in an HDF file. Therefore, it is not safe to assume that a reference number is the index of an image.

	FORTRAN	integer function d2rref(filename, ref)
		character*(*) filename
		integer ref