sd_id	IN:	SD interface identifier returned by SDstart
	sds_index	IN:	Index of the data set

	Purpose	Obtains the data set identifier (sds_id) of a data set.
	Return value	Returns the data set identifier (sds_id) if successful and FAIL (or -1) otherwise.
	Description	SDselect obtains the data set identifier (sds_id) of the data set specified by its index, sds_index.
		The integration with netCDF has required that a dimension (or coordinate variable) is stored as a data set in the file. Therefore, the value of sds_index may correspond to the coordinate variable instead of the actual data set. Users should use the routine SDiscoordvar to determine whether the given data set is a coordinate variable.
		The value of sds_index is greater than or equal to 0 and less than the number of data sets in the file. The total number of data sets in a file may be obtained from a call to SDfileinfo. The SDnametoindex routine can be used to find the index of a data set if its name is known.

	FORTRAN	integer function sfselect(sd_id, sds_index)
		integer sd_id, sds_index