file_id	IN:	File identifier returned by Hopen
	tag	IN:	Tag of the data element to add or replace
	ref	IN:	Reference number of the data element to add or replace
	data	IN:	Pointer to data buffer
	length	IN:	Length of data to write

	Purpose	Writes a data element or replaces an existing data element in a HDF file.
	Return value	Returns the number of bytes written if successful and FAIL (or -1) otherwise.