More Transform/Compression Examples

[ax3l]

We need a bit more transform/compression examples in both the manual and example codes.

I also realized that we can set Dataset::setCustomTransform which is only handled in ADIOS1 right now.
And the Dataset::setCompression options handled in ADIOS2 are a bit too simple (name + level) for the complex blosc and zfp parameters.

For defaults, the ADIOS2 implementation for compression/operators is still incomplete: https://openpmd-api.readthedocs.io/en/0.12.0-alpha/details/backendconfig.html#configuration-structure-per-backend

See also open to do points in: #569

[ax3l]

Missing manual example:

ZFP Compression

With

std::string options = R"END(
{
  "adios2": {
    "dataset": {
      "operators": [
        {
          "type": "zfp",
          "parameters": {
              "precision": "3"
          }
        }
      ]
    }
  }
}
)END";

As of 0.12.0, this only works with ADIOS2 (2.6.0) right now and is applied to all data sets.
This is the same precision (fixed-precision mode with three uncompressed bits per scalar) as in A. Huebl et al., ISC'17.

Note: As of ADIOS 2.6.0, an upstream bug prevents us from using aggregation together with operators/transformations.