sd_id	IN:	SD interface identifier returned by SDstart
	sds_ref	IN:	Reference number of the data set

	Purpose	Returns the index of a data set given the reference number.
	Return value	Returns the index of the data set (sds_index) if the data set is found and FAIL (or -1) otherwise.
	Description	SDreftoindex returns the index of a data set identified by its reference number, sds_ref.
		The value of sds_index returned by SDreftoindex can be passed to SDselect to obtain a data set identifier (sds_id).

	FORTRAN	integer function sfref2index(sd_id, sds_ref)
		integer sd_id, sds_ref