This pleasant-looking group of Americans has lived history, all right: one presidential death from illness, one presidential assassination, one presidential resignation. They got together recently at a play in Washington's Kennedy Center. They are, from left, Margaret Truman Daniel, President Ford, his daughter Susan, Lynda Bird Johnson Robb, her husband Charles, David Eisenhower, his wife Julie Nixon Eisenhower and Betty Ford. The President was ebullient: he had made up his mind to pardon Richard Nixon and the storm of criticism was yet to fall. Did Julie Eisenhower know about the pardon too? Probably. If so, she had special reason for the look of profound gratitude that she bestowed upon Gerald Ford that night.