By Amanda DeMarco STUTTGART: Michael Zöllner isn’t sentimental about his days as an independent publisher. He and his partner Tom Kraushaar led Tropen Verlag autonomously until January 2008, when it merged with the larger Stuttgart-based Klett-Cotta Verlag. Tropen, which published 10 to 12 titles per year, became an imprint of Klett-Cotta and the two became CEOs of Klett-Cotta’s list and …