Roadmap: Wishlist of new features

A wishlist of features after the upcoming releases:

## Near-Term

- [ ] Implement [openPMD 2.0](https://github.com/orgs/openPMD/projects/4)
- [ ] https://github.com/openPMD/openPMD-standard/issues/142
- [ ] Custom hierarchies
see description https://github.com/openPMD/openPMD-standard/issues/115
- [x] New ADIOS2 schema with support for modifiable attributes
https://github.com/openPMD/openPMD-api/pull/1310
- [ ] non-MPI parallel reads in DASK with ADIOS https://github.com/ornladios/ADIOS2/pull/3651
- [x] New ADIOS2 JoinedArray support for particles #1374 #1382
https://github.com/ornladios/ADIOS2/pull/3466
- [x] Deprecation and later removal of `RecordComponent::SCALAR`
https://github.com/openPMD/openPMD-api/pull/1154
- [ ] Full support for steps in ADIOS2, full support for variable-based iteration encoding. This implies:
* Default enabled steps
* Support random-accessing steps
* Skip duplicate iterations at read time
* Support reopening closed iterations #1606
- [ ] Julia bindings #1025
- [ ] better object model for default attributes #1439
- [ ] distributed initialization of a read-access dataset in ADIOS2 within non-MPI contexts (e.g. DASK, ref https://github.com/ornladios/ADIOS2/pull/3651)
- [ ] `openpmd-pipe`: modularize into visitor pattern
- [ ] `openpmd-pipe`: reuse in new CLI tools such as `openpmd-coarsen` (fields, particles #1390)

## Mid-Term

* Support for joined arrays in backends other than ADIOS2
* `async for iteration in s.read_iterations()` for Python
* More flexible reads in C++17 #1372 and Python
* MPI-wise logging of IO actions
* Performance optimization: Long-running simulations (many iterations, reading and writing)
* Specify default attributes not upon construction, but upon closing, clean up the logic for specifying defaults, constructors and destructors of our object model
* Context: crashing simulations
* If in read, a standard attribute is missing, then warn and add a reasonable default (e.g., a 3-value axisLabel for a 3D-mesh)
* Support for PIConGPU-style dataset-specific JSON/TOML configuration
https://picongpu.readthedocs.io/en/0.6.0/usage/plugins/openPMD.html#cfg-file, also for iteration-specific configuration, e.g. for InitialBufferSize per file
* *Maybe* Flag for writing attributes only from rank 0
* Python docstrings #1328
* *Maybe* SoA <-> AoS flexibility (affects the standard)
* Probably requires struct-type fields
* Project structure: Separate MPI headers from serial headers
With this change: Provide openPMD-api via Linux package managers
* Compression and plugins in HDF5
* HDF5 hardlinks + maybe as a fallback softlinks https://github.com/openPMD/openPMD-standard/issues/283

# Long-Term / Ideas

* Synchronous mode: Avoid UB for store and load chunks
* In both C++ and Python, we would like to avoid that the user can interact with allocated but non-flushed (UB) data.
* For this, we could rename `storeChunk` / `loadChunk` to `...Async()`, which returns a `std::future` (C++) or `asyncio.Future` (Python).
* We need to keep track which of these in-flight objects we created and will set them to valid on `series.flush()`.
* If a future is awaited before flush was called, we throw a runtime exception, which allows to recover in interactive use. Futures also allow us to check if the futures are valid w/o having to catch exceptions.
* The existing APIs would be sync.

* *Maybe* Use ADIOS2 group feature in reading (-> faster parsing)
* *Maybe* Chunk distribution algorithms
* *Maybe* Async I/O (especially Python)