Add Parallel I/O Infrastructure

[SeanBryan51]

This change brings in the interface layer for working with the NetCDF Fortran and ParallelIO (PIO) libraries in CABLE. PIO allows for each MPI rank to participate in I/O operations collectively and is a first step in adding MPI support to the serial offline driver, and eventually, to replace the legacy MPI implementation (#358).

To keep CABLE dependencies as minimal as possible for running in serial mode (without MPI), the interface layer is designed such that PIO support is optional.

To build CABLE with PIO, PIO version 2.6.8 or greater is required.

Type of change

Please delete options that are not relevant.

• New feature
• 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.

2026-03-17 12:43:56,608 - INFO - benchcab.benchcab.py:380 - Running comparison tasks...
2026-03-17 12:43:56,634 - INFO - benchcab.benchcab.py:381 - tasks: 168 (models: 2, sites: 42, science configurations: 4)
2026-03-17 12:46:38,124 - INFO - benchcab.benchcab.py:391 - 0 failed, 168 passed

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📚 Documentation preview 📚: https://cable--706.org.readthedocs.build/en/706/

[SeanBryan51]

Hi @Whyborn, now that #700 has been merged, these changes are ready to go in. Do you mind giving this your review?

[Whyborn]

A lot of the internal functions are missing any commentary. I found a lot of the routines not particularly intuitive, so some commentary explaining human-readably what the routines are doing would be a big help I think. Doesn't have to be public facing documentation, more like internal developer comments.

I find the tests not very "human"- a lot of abstractions, that I didn't find very easy to reason about exactly what they're doing. This could probably be solved by some commentary as well.

[SeanBryan51]

Thanks for taking a look Lachlan, I forgot to add docs to cable_netcdf_internal.F90. I realised cable_netcdf_internal.F90 was only really necessary for initialising the I/O handler, so I've trimmed down that file to only include the I/O handler initialisation and renamed it to cable_netcdf_init.F90 in 0f30366.