IObj.py: consider splitting

[PMeira]

Not essential or urgent: IObj is so big that Sphinx has trouble parsing it!

Thankfully we can use PyPy to accelerate that (still takes several minutes), but this makes it unlikely we would be able to rebuild the docs on CI. Locally we can reuse the cache and it only that long in the first run.

I guess we can consider splitting after a more final version is available.

[PMeira]

We'll probably move the code related to IObj to the new (AltDSS?) repo in a couple of weeks before splitting.