Skip interaction when src/trg octant is empty

- For particale FMM, interactions (V,X,W-list) are skipped if either the
  source octant has no source points or the target octant has no target
  points.

- New vectorized kernels for Laplace double-layer and gradient.

- src/profile.cpp: In Add_FLOP(..) and Add_MEM(..), replace "omp
  critical" with "omp atomic".