This work was supported in part by the National Science Foundation (CNS-0953447 and CNS-0905308). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. This work was also supported in part by the US Department of Energy, National Nuclear Security Administration, and Advanced Simulation and Computing Program, as a Cooperative Agreement under the Predictive Science Academic Alliance Program, under Contract No. DE-NA0002378.