sixtracklib/cuda: rewrite C++11 ideoms in the cuda_env.cu implementation that prevented successful compilation under cuda 7.5
sixtracklib/cuda: rewrite C++11 ideoms in the cuda_env.cu implementation that prevented successful compilation under cuda 7.5

SixTrack/SixTrackLib,SixTrack/SixTrackLib,SixTrack/SixTrackLib,SixTrack/SixTrackLib