filename	IN:	Name of the HDF file
	ref	IN:	Reference number for next add or put operation

	Purpose	Specifies the reference number, ref, of the dataset to be overwritten next by DFSDputdata or DFSDadddata.
	Return value	Returns SUCCEED (or 0) if successful and FAIL (or -1) otherwise.
	Description	DFSDwriteref verifies the refence number's existence before returning. If a non-existent reference number is specified, an error code will be returned.
		As this routine alters data in a destructive manner, DFSDwriteref should be used with caution.

	FORTRAN	integer function dswref(filename, ref)
		character*(*) filename
		integer ref