SMC Storage is not MPI-compatible

[peleser-nasa]

Currently requires something like:

    with HDF5Storage(H5_FILE, "a"): # load up state on all cores
        smc = AdaptiveSampler(kernel)
    with HDF5Storage(HIERARCH_H5_FILE, "a") if comm.Get_rank() == 0 else nullcontext():
        steps, mll = smc.sample(
            N_PARTICLES, N_MCMC_SAMPLES, target_ess=0.9
        )

Whereas we would prefer save_step to be rank0 only so that we can use context as normal:

    with HDF5Storage(H5_FILE, "a"):
        smc = AdaptiveSampler(kernel)
        steps, mll = smc.sample(
            N_PARTICLES, N_MCMC_SAMPLES, target_ess=0.9
        )