Add example using MPIClusterManager

[vchuravy]

using MPI
using MPIClusterManagers
using LAMMPS

MPI.Init()

manager = MPIClusterManagers.start_main_loop(MPI_TRANSPORT_ALL) # does not return on worker

cmds = [
    "clear",
]

# do_n_tasks_in_parallel

function run_lammps(manager, cmds)
    @mpi_do manager begin
        LMP(["-screen", "none"], MPI.COMM_WORLD) do lmp
            @test LAMMPS.version(lmp) >= 0
            for cmd in cmds
                command(lmp, cmd)
            end
        end

        # data post-processing
    end
end