Simplify LJ parameter lookup

 Simplify LJ parameter lookup

  - Unify the CUDA per-pair scaled C6/C12 LJ parameter
    loading codepaths so both texture and LDG paths do
    8-byte float2 loads.
  - Unify the CUDA per-atom LJ parameter loading codepaths
    so both texture and LDG paths do 8-byte float2 loads.
  - Simplify the LJ self interaction energy term calculation
    by only using the non-texture loads path to simplify code.
    As this is outside the main loop, its impact is negligible.
  - Use float2 loads for the above in OpenCL as well.