Officials of the 56-year-old Harbor Club on Thursday notified members and employees that the private club at the top of the Norton Building in downtown Seattle is closing at the end of the year. The decision to close was “very difficult,” said club President Bob Carlile, who added that members “tried, quite frankly, everything” to keep the august organization going but were unable to overcome changing social habits. The club opened along with the Norton Building at the dawn of the “Mad…