Written by DCist contributor Dave Weigel The Magnetic Fields have a song called “Washington, D.C.” It’s not actually about Washington, per se. The city is a collection of incidental, touristy, democratic details, and what makes it important to vocalist Claudia Gonson is that it’s “where my baby lives, that’s all.” So it’s fitting that the band put the song together as an afterthought. Gonson walked on the stage for an encore, promised the audience an…