Add parallel I/O output system

[SeanBryan51]

This change brings in a new output system based on the parallel I/O infrastructure introduced in #706, and is a direct replacement of the previous output module used for offline CABLE. The main motivation behind this is to add MPI support to the serial offline driver, and eventually, to replace the legacy MPI implementation (#358). This also makes progress towards the proposed output redesign (#715) by introducing the underlying data structures needed for its implementation.

The new output system brings in the following enhancements:

• Optional parallel I/O support via PIO
• A new "aggregator" class for calculating time aggregations throughout a simulation
• New data structures for describing output variables which allows for easily adding new diagnostics and/or restarts variables

This change should be brought in after #712.

Type of change

Please delete options that are not relevant.

• Enhancement
• New or updated documentation

Checklist

• The new content is accessible and located in the appropriate section
• I have checked that links are valid and point to the intended content
• I have checked my code/text and corrected any misspellings

Testing

• Are the changes bitwise-compatible with the main branch? If working on an optional feature, are the results bitwise-compatible when this feature is off? If yes, copy benchcab output showing successful completion of the bitwise compatibility tests or equivalent results below this line.

CABLE benchcab runs tested using ifort 2021.10.0.

2026-04-28 06:43:59,566 - INFO - benchcab.benchcab.py:380 - Running comparison tasks...
2026-04-28 06:43:59,593 - INFO - benchcab.benchcab.py:381 - tasks: 168 (models: 2, sites: 42, science configurations: 4)
2026-04-28 06:46:47,366 - INFO - benchcab.benchcab.py:391 - 0 failed, 168 passed

Please add a reviewer when ready for review.

---

📚 Documentation preview 📚: https://cable--713.org.readthedocs.build/en/713/

[SeanBryan51]

Hi @abhaasgoyal @Whyborn, thank you for your time in reviewing these PRs! This one is now ready for review, and no rush of course, it's the heaviest one so far!

As an introduction, it might be useful to read these developer documentation pages:

• https://cable--713.org.readthedocs.build/en/713/api/module/cable_output_mod.html
• https://cable--713.org.readthedocs.build/en/713/api/module/aggregator_mod.html

Benchcab is running currently - I had to make a small change to achieve bitwise compatibility, I will update the status soon.

Thank you again!