Fix h5 early chunk improved iteration open

[franzpoeschel]

This refactors and centralizes the routines for opening an iteration and its files, making it accessible for usage elsewhere. Use in availableChunks().

TODO:

• In-line documentation
• Documentation as mentioned in availableChunks open logic (HDF5: Early Chunk Read) openPMD/openPMD-api#1035
• Possible logic bug that I'll need to check: what does deferred parsing do in READ_WRITE mode?

[ax3l]

@franzpoeschel nice, thanks a lot! Shall I leave this open as you work on the to-do's? The overall concept looks good 👍

ICC seems to hang in 3/4 Test #3: Serial.SerialIO ..................***Timeout 1500.07 sec atm still.

[franzpoeschel]

@franzpoeschel nice, thanks a lot! Shall I leave this open as you work on the to-do's? The overall concept looks good +1

I'll update the documentation today, then I'd say we can merge this back with the original PR, so you won't have to push the CI manually each time (as long as you don't mind that I will then push to your branch).

ICC seems to hang in 3/4 Test #3: Serial.SerialIO ..................***Timeout 1500.07 sec atm still.

There's been a typecasting bug, maybe that was the reason.

[franzpoeschel]

There's no central point in the documentation that documents Iteration::open() since the documentation is sorted not by API accesses, but by topic. It is mentioned at the two places where it is relevant:

• In backendconfig.html to explain its interaction with defer_iteration_parsing.
• In mpi.html to explain its interaction with collective file accesses.

I've added a note to streaming.html that it makes explicit use of Iteration::open() obsolete and extended the Doxy documentation to note all three use cases.

[franzpoeschel]

Can you let the CI run again @ax3l? Otherwise, I think this is ready to go back to the original PR.

[ax3l]

Nicely done, thank you! 🚀 ✨