file_id	IN:	File identifier returned by Hopen
	tag	IN:	Tag of the data element
	ref	IN:	Reference number of the data element

	Purpose	Returns the length of a data object specified by the tag and reference number.
	Return value	Returns the length of data element if found and FAIL (or -1) otherwise.
	Description	Hlength calls Hstartread, HQuerylength, and Hendaccess to determine the length of a data element. Hlength uses Hstartread to obtain an access identifier for the specified data object.
		Hlength will return the correct data length for linked-block elements, however it is important to remember that the data in linked-block elements is not stored contiguously.