Genarlize pack and unpack
Generalize the function
- void Cell_Container::pack(std::vector<Cell*> *all_cells, mpi_Environment &world, mpi_Cartesian &cart_topo)
- void Cell_Container::unpack(mpi_Environment &world, mpi_Cartesian &cart_topo)
which contain an if the cell moves left or right. Pass the direction as an argument instead