Back in the 1960s, Thomas Nystrom was the frontman for a nationally known rock band which once opened for the Beatles. These days, Nystrom is a 58-year-old music aficionado who has spent much of his time lately battling colon cancer. But yesterday, the old Minneapolis rocker fulfilled a dream which he swears was every bit as thrilling as opening for the Fab Four. As an audience of friends and relatives clapped, stomped, and cheered, Nystrom mounted a podium at Minneapolis’s Orchestra Hall in full tux and tails, picked up a baton, and led his other favorite band, the Minnesota Orchestra.