USER-DPD: move nstencil_ssa out of core LAMMPS into USER-DPD

src/USER-DPD/npair_half_bin_newton_ssa.cpp

src/USER-DPD/nstencil_half_bin_2d_newton_ssa.cpp

src/USER-DPD/nstencil_half_bin_2d_newton_ssa.h

src/USER-DPD/nstencil_half_bin_3d_newton_ssa.cpp

src/USER-DPD/nstencil_half_bin_3d_newton_ssa.h

src/USER-DPD/nstencil_ssa.h

src/npair.cpp

src/npair.h

src/nstencil.h