Append mode: Option to not truncate files in file-based iteration encoding #1630
Description
For boosted frame simulations, to implement checkpoint-restart workflows with the file-based, multi-steps-per-file output in #1629, we want to support to continue writing to an iteration after a checkpoint-restart interruption of an application.
We need an append mode in file-based iteration encoding that does not truncate files (openPMD iterations) that were already partially written.
As this is a very specific use case, adding a short-circuit style logic should be sufficient for this, e.g. Series("output_%T.bp5", Access::APPEND, R"(adios2.engine.open_mode = "append")" to override the ADIOS2 backend's access mode for ADIOS files. This would keep our frontend logic cleaner.
X-ref: #1629