Decide long-term handling of skipped slow notebook tests

[georgii-tishenin]

PR #699 skips several long-running notebook tests to reduce CI runtime.

The following notebooks were marked as skipped:

• examples/Notebooks/Circuits/EMT_Ph3_IEEE9_SSN_InverterMix.ipynb
• examples/Notebooks/Performance/EMT_IEEE_39bus_Multi_Diakoptics.ipynb
• examples/Notebooks/Components/EMT_Ph3_GFL_Comparison.ipynb
• examples/Notebooks/Performance/DP_WSCC9bus_SGReducedOrderVBR_SparseLU_KLU_fault_comparison.ipynb
• examples/Notebooks/Components/Syngen_9Order_DCIM_VBR_Governor_Exciter.ipynb
• examples/Notebooks/StateSpace/DP_Ph1_Inverter_StateSpaceExtraction.ipynb
• examples/Notebooks/StateSpace/EMT_Ph3_Inverter_StateSpaceExtraction.ipynb

This is a practical short-term solution, but we should decide the long-term handling for each notebook.

Possible options:

1. Keep the notebook as it is and leave it skipped in CI.
2. Split it into two notebooks:
   • a lightweight validation notebook that runs in CI;
   • a longer demonstration notebook that remains skipped.
3. Reduce the runtime so the notebook can stay in CI, for example by reducing the simulation duration, increasing the time step, simplifying parameter studies, or limiting plotted/validated cases.

The goal is to keep CI reasonably fast while still preserving useful examples and validation coverage.

[leonardocarreras]

Hi Georgii!
Thanks for thinking about this... I support long-term splitting the concerns and not using the notebooks as tests and only as examples.

My opinion is that we should move the testing in two different directions:

1. Test the components in python files (with code coverage on)

• Have some unit tests
• Have some networks that use most of the components
• Do some few villas testing

2. Do some of the test that run long experiments in python or c++ (with assertions but code coverage off)

I think also we should separate the notebooks from the testing because there we do things like plotting which is generally not needed for a machine test, but is nice as an example for users.