libPMacc: implement error function math::erf()

Implements the [CUDA error function](http://developer.download.nvidia.com/compute/cuda/4_1/rel/toolkit/docs/online/group__CUDA__MATH__SINGLE_g3b8115ff34a107f4608152fd943dbf81.html#g3b8115ff34a107f4608152fd943dbf81) in `math::erf()` of libPMacc.