tokens: The authors thank I. Cooke, R. Martin-Domenech, P. Maksiutenko, and L. Kelley for valuable feedback. J.B.B. acknowledges funding from the National Science Foundation Graduate Research Fellowship under grant DGE1144152. This work was supported by an award from the Simons Foundation (SCOL # 321183, K.O.). Software: NumPy (Van Der Walt et al. 2011 ), Matplotlib (Hunter 2007 ), emcee (Foreman-Mackey et al. 2013 ).
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