Refactor Jacobian assembly

[kakeueda]

Description

Jacobian assembly currently uses a COO matrix, and we sort and rebuild CSR row pointers each step for IDA. The overhead from this process can become a bottleneck as nnz grows. This change keeps the system Jacobian in CSR and preserves a COO→CSR mapping for direct CSR updates.

Closes #301

cc @nkoukpaizan

Proposed changes

• Store the system Jacobian in CSR format in PowerElectronicsModel.
• Update allocate() to populate CSR entries and preserve the mapping.
• Add CsrJac for the solver configuration (PhasorDynamics still uses the Jac function).

Currently PowerElectronics only.

Checklist

• All tests pass.
• Code compiles cleanly with flags -Wall -Wpedantic -Wconversion -Wextra.
• The new code follows GridKit™ style guidelines.
• There are unit tests for the new code.
• The new code is documented.
• [N/A] The feature branch is rebased with respect to the target branch.
• I have updated CHANGELOG.md to reflect the changes in this PR. If this is a minor PR that is part of a larger fix already included in the file, state so.

[kakeueda]

Timed runSimulation(t_final) in the microgrid app. CSR assembly improved runtime, while the error remained unchanged.

Before

After

[shakedregev]

Still need to test this. Looks good except for naming inconsistencies.

[shakedregev]

Approved pending fixes to variable names

[shakedregev]

Timed runSimulation(t_final) in the microgrid app. CSR assembly improved runtime, while the error remained unchanged.

Before

After

[shakedregev]

It's impressive that there are such substantial speed gains on even a small system. This means that for real systems it's probably astronomical. I wonder if we can test that.

[pelesh]

• CSR matrix implementation should remain in the *.cpp file.
• I would avoid using STL containers here because we want to prototype parallel implementations.
• Do not use existing COO matrix implementation in GridKit. If needed, port COO matrix from Re::Solve.

I made some other (mostly nitpicking) comments about the code.

[nkoukpaizan]

Excellent work!

I only have a few minor comments.

One last thing is that there are some calls to sysmodel.getJacobian().printMatrix() in examples/PowerElectronics that will no longer be effective. It would be good to update to print the CSR matrix instead. We can always updated again when we remove the temporary getCsrJacobian method.