file_id	IN:	File identifier returned by Hopen
	length	IN:	Length of the new element

	Purpose	Specifies the length of a new HDF element.
	Return value	Returns SUCCEED (or 0) if successful and FAIL (or -1) otherwise.
	Description	This function can only be used when called after Hstartaccess on a new data element and before any data is written to that element.