GPU accelerated population annealing algorithm

For the default parameter values used in the sample programs, L=64, θ=100, β0=0, βf=1, Δβ=0.005, R=20000, a typical run time on an NVIDIA Tesla K80 GPU is 151 seconds for the single spin coded (SSC) and 17 seconds for the multi-spin coded (MSC) program (see Section 2 for a description of these parameters).