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.