Enabling compression for the ADIOS2 backend

[eschnett]

I want to use openPMD with ADIOS2 as back-end, and I want to enable ADIOS2 compression via the blosc library. How do I do this?

When I create an ADIOS2 file directly the I would write approximately this:

      const adios2::Operator compressor =
          adios.DefineOperator("Blosc2Compressor", adios2::ops::LosslessBlosc);
      // Use a high compression rate and a bitshuffle filter
      const adios2::Params compressor_options{
          {adios2::ops::blosc::key::clevel, adios2::ops::blosc::key::clevel_9},
          {adios2::ops::blosc::key::doshuffle,
           adios2::ops::blosc::key::doshuffle_bitshuffle},

...

                    adios2::Variable<CCTK_REAL> var =
                        io.DefineVariable<CCTK_REAL>(varname, {}, {},
                                                     {1, 1, 1});
                    var.AddOperation(compresser, compresser_options);

[franzpoeschel]

Hey Erik,
these parameters are passed via JSON/TOML, see e.g. here for zlib or here for Blosc.
You can specify the default compression by passing the parameter to the Series constructor or a dataset-specific compression, by setting this JSON value in the Dataset class (as passed to resetDataset().

Check also the documentation.