“ The claim that the decedent's advance in grade and pay is to be upon the grade to which he had been advanced without additional pay is without merit. To concede it would be to conclude that Congress intended to advance him not upon the grade he had at retirement, but upon the gratuitous advancement; and that Congress purposed to advance him one other step over that which he had at retirement, and two steps in pay. The basis of the gratuity of Congress was the grade and pay at retirement. ”