Add AtomPack grid storage and MPI neighbor prototype tests

[Yanglijf]

Summary

• Added AtomPack SoA data container for neighbor-search atoms
• Added GridStorage flat box storage with prefix-sum filling
• Added UnitCell -> AtomPack construction for PBC and non-PBC cases
• Added MPI ghost record append path into AtomPack
• Added MPI neighbor prototype test
• Added OpenMP consistency coverage for current Cell-List path
• Fixed MPI test lifecycle by explicitly finalizing MPI

Tests

• MODULE_CELL_NEIGHBOR_sltk_grid
• MODULE_CELL_NEIGHBOR_atom_pack
• MODULE_CELL_NEIGHBOR_mpi_domain
• MODULE_CELL_NEIGHBOR_mpi_neighbor_proto
• mpirun --mca btl ^tcp -np 2 MODULE_CELL_NEIGHBOR_mpi_domain
• mpirun --mca btl ^tcp -np 2 MODULE_CELL_NEIGHBOR_mpi_neighbor_proto