OF NATURAL HISTORYAn avant metal spectacle of the most obscene, obese and obstinate, Sleepytime Gorilla Museum sound like a brigade of skeleton soldiers marching through post industrial cities decorated by the finest of silken banners to cover up the decay. Singer Nils Frykdahl can croon with a baritone even deeper than Simon Bonney, with the same sense of dark philosophy, as he does on the opening "Hymn to the Morning Star." But he's just as likely to bark like James