The dear old gentleman minding his grandchildren on a winter's afternoon and the bombastic summer dictator blasting Casey Stengel and Yogi Berra are, incredibly, one and the same: Bill Summers, baseball's most experienced umpire. When Summers called his first major league game, Mickey Mantle was an infant about the size of the young man cradled in grandpa's arms. This coming season will mark Summers' quarter-century in the majors, a tribute to the umpire's stock in trade which Summers has always possessed in abundance: good health, encyclopedic knowledge of the game, complete self-control and an unlimited source of truculent, dogmatic, unyielding self-confidence.