LAST FRIDAY EVENING, a sudden commotion interrupted the opening of “Alex Katz: Gathering” at the Guggenheim Museum.It was not a protest or a stunt. Just as outgoing director Richard Armstrong informed the New York Social Diary photographer Jill Krementz that the ninety-five-year-old artist was not expected to appear, he materialized seemingly out of nowhere on the bottom ramp of the rotunda. Once spotted, the sound of applause and cheers erupting in the lobby gained decibels as the hundreds of people on the upper tiers joined a spontaneous demonstration right out of Hello, Dolly!Looking swell