Initial Implementation of Subsystem Model for Partition Simulation

[abdourahmanbarry]

Description

In this PR, we introduce the initial implementation of the SubsystemModel for partitioned Power Electronics simulation. The implementation provides the infrastructure required to partition Power Electronic networks, and to evaluate subsystem residuals and Jacobians independently. This lays the foundation for implementing co-simulation methods in future pull requests.

Proposed changes

• We added a SubsystemModel class to represent an individual partition and a BusPartitionInterface component to mark partition boundaries.
• Implemented residual and Jacobian evaluation for subsystem models.
• Added support for user-defined coupling functions required for solving subsystems with multi-stage co-simulation methods.

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.
• 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.