Groupbased encoding with ADIOS2 steps shows data from the first iteration only

[franzpoeschel]

Noticed first by @pordyna

This is more an openPMD-api issue, affecting the openPMD-viewer. The viewer can now read from a groupbased openPMD Series as well. Your implementation uses available_chunks(), which currently only returns the chunks of the currently active IO step – or in case IO steps are not used, like in the openPMD-viewer, the chunks of the first IO step. Consequence: Only data from the first iteration is visualized.

openPMD/openPMD-api#1291 fixes this behavior, I plan this as part of the next release.
It introduces a new access type which helps distinguish random-access from step-for-step reading workflows. For random-access workflows, available_chunks() now returns chunks from all steps.

There is no need to change anything in the openPMD-viewer, other than expecting the fix from the openPMD-api.