Non-kokkos candidate of PairExp6rxKokkos::getMixingWeights to improve vectorization on the KNL.

- Moved the particle loop inside a replica of getMixingWeights, getMixingWeightsVect,
  and refactored to improve vectorization.
- Added OMP SIMD and OMP threading directly inside that function but will replace with
  kokkos parallel_for and parallel_reduce methods later.