1. Department of Statistical Science, Duke University, Durham, NC 27708-0251 USA.
2. Joint first authors.
3. H. Milton Stewart School of Industrial & Systems Engineering, Georgia Institute of Technology, Atlanta, GA 30308 USA.
4. Department of Physics, Duke University, Durham, NC 27708 USA.
5. Department of Physics and Astronomy & Department of Mathematics, Vanderbilt University, Nashville, TN 37240 USA.
6. Corresponding author. Department of Statistical Science, Duke University, Durham, NC 27708-0251 USA.