The cast of Signature’s “Showboat.” Signature Theatre’s production of Showboat demonstrates two things: that sometimes a hefty dose of an old-school musical is just what you need, and that even those old chestnuts can still have some surprises in store. Take Terry Burrell’s rendition of “Bill,” Jerome Kern and Oscar Hammerstein’s classic ode to inexplicable love (I remember the lyric, “I love him because he’s…I don’t know,” getting a half hour-long rave from my…