Feature: add MPI communicator injection for general-purpose parallel workflows

[dyzheng]

Replace hardcoded MPI_COMM_WORLD with configurable base_comm parameter in the MPI initialization path, enabling workflows like parallel NEB, phonon calculations, or replica exchange MD to split MPI ranks into independent groups, each running its own ESolver instance in a separate subprocess.

Changes:

• parallel_global: add base_comm parameter to divide_pools(), split_diag_world(), split_grid_world(), init_pools()
• parallel_reduce: add comm parameter to all reduce_all, reduce_min, reduce_max, reduce_double_allpool, gather_int_all functions
• parallel_common: add comm parameter to all bcast_* functions
• PyESolverLCAO: add mpi_comm_handle parameter to initialize()
• LCAOWorkflow: add mpi_comm parameter forwarded to ESolver

All parameters default to MPI_COMM_WORLD for full backward compatibility. Existing code compiles and behaves identically without modification.

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• Have you explained the changes of codes in core modules of ESolver, HSolver, ElecState, Hamilt, Operator or Psi? (ignore if not applicable)

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