Use CUDA float3 vector type instead of fvec in GPU bonded kernels

Use CUDA float3 vector type instead of fvec in GPU bonded kernels

Using fvec in CUDA can be inneficient and requires re-definition
of vector operations. This commit switches the GPU bonded kernels
to using float3 instead of fvec.

Change-Id: Ib3bfa3e20501cdd22fc6e599b749c750fa468e80