fix #472 add workaround for cuda 5.5+ bug 

- since cuda 5.5  nvcc generate wrong loops in some cases
- this workaround fix wrong current calculation with cuda 5.5+

I opened a  topic in the cuda forum at:
https://devtalk.nvidia.com/default/topic/752200/cuda-programming-and-performance/nvcc-loop-bug-since-cuda-5-5/

Tested with:
- [x] cuda 6.0 KHI positrons/electrons CIC Esirkepov 3D
- [x] cuda 6.0 KHI positrons/electrons CIC Esirkepov 2D
- [x] cuda 5.0 KHI positrons/electrons CIC Esirkepov 2D