add "atomonly" optimized neighbor list build styles to USER-OMP

src/USER-OMP/npair_full_bin_atomonly_omp.cpp

src/USER-OMP/npair_full_bin_atomonly_omp.h

src/USER-OMP/npair_half_bin_atomonly_newton_omp.cpp

src/USER-OMP/npair_half_bin_atomonly_newton_omp.h