How to roughly control ADIOS2 block number and WarpX particle number

[Change72]

1. ADIOS2 block number:

I want to increase the ADIOS2 block number. I notice the field number is matched with grids number. For example, key = /data/600/fields/E/x. However, for particle blockNum, it seems a magic number: key = /data/600/particles/electrons/momentum/x. I didn't find any parameter to control it, and I guess there should be one in the openPMD layer.

n_cell	max_grid_size	blocking_factor	grids	field blockNum	particle blockNum	smallest grid	biggest grid
32x32x256	64	32	4	4	128	32x32x64	32x32x64
256x256x512	64	32	128	128	128	64x64x64	64x64x64
64x64x256	64	32	4	4	4	64x64x64	64x64x64
256x256x256	8	8	32768	32768	738	8x8x8	8x8x8

    bpIO = adios.DeclareIO("ReadBP");
    bpIO.SetParameter("Threads", std::to_string(nThreads));
    bpReader = bpIO.Open(bpFileName, adios2::Mode::Read);

    adios2::Variable<double> x_meta_info = bpIO.InquireVariable<double>(key + "x");
    auto xBlocksInfo = bpReader.AllStepsBlocksInfo(x_meta_info);
    auto x_it = xBlocksInfo.begin();
    
    for (; x_it != xBlocksInfo.end(); ++x_it) {
        const auto &var_vec1 = x_it->second;
        std:cout << var_vec1.size() << std::endl;  // as ADIOS2 block number
    }

2. for the particle number in bpls, the injection style I used is electrons.injection_style = "NRandomPerCell". The first two lines satisfy the exception, but the third and fourth lines are quite weird.

n_cell	iteration	num_particles_per_cell	num_particles_in_last_iteration
32x32x256	500	1	116072
32x32x256	500	100	11607258
256x256x512	600	100	14069550
64x64x256	600	10000	45020389
256x256x256	600	1	64950