Ensure ADIOS2 Variable Based Encoding Can be Read

[ax3l]

We need to check if variableBased encoding works for ADIOS2 BP5 files in openPMD-viewer.

Orthogonal approaches, I would say we should support both:

• Support random access with Random-access for variable-based encoding (i.e. use SetStepSelection for ADIOS2 steps) openPMD-api#1706 - we need to benchmark the performance impact of this.
• Add a generator-like expression to iterate over all particles/fields, as in the streaming read mode. This needs new user-facing APIs in openPMD-viewer.

Docs:
https://openpmd-api.readthedocs.io/en/0.16.1/backends/adios2.html

cc @ax3l @guj @franzpoeschel

[franzpoeschel]

#1706 is nearly feature-complete, so this could be tested already. Currently missing is the support for modifiable attributes which will come as a follow-up PR (this requires pre-parsing the ADIOS2 file in adios2::Mode::Read instead of adios2::Mode::ReadRandomAccess and is hence not entirely trivial). Consequences:

1. Information such as /data/time will currently be useless.
2. Data Series with different contents per Iteration will not be supported since we would have to resolve file steps to variable steps. Could maybe be done once preparsing is implemented, but it's not a priority.

Add a generator-like expression to iterate over all particles/fields, as in the streaming read mode. This needs new user-facing APIs in openPMD-viewer.

I didn't expose it for Python yet since the final API is still up for discussion, but note that on the current dev and especially with #1706, series.iterations, series.writeIterations() and series.readIterations() can all be replaced by the unified API series.snapshots().