REPCA Documentation + Implementation

[lukelowry]

Description

Implementation and documentation for the REPCA PhasorDynamics converter model.

Proposed changes

• Added REPCA model implementation, documentation, and diagram.
• Added REPCA JSON parsing and SystemModel assembly support.
• Added focused PhasorDynamicsConverterRepcaTest unit coverage.
• Added minimal REGCA pbranch/qbranch feedback exports for REPCA plant feedback.
• Updated converter registration, input-format docs, and changelog.

Checklist

• All tests pass (except known PE failure)
• 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.

Further comments

None

[pelesh]

This looks good to me and ready to merge.

Reviews from @nkoukpaizan and @abirchfield would be helpful before merging.

[lukelowry]

@nkoukpaizan @abirchfield good to go!

[abirchfield]

This is good.

[nkoukpaizan]

Overall looks good to me and tests pass. I would just ask for some additional alignment of the implementation, particularly in the tests.

[lukelowry]

Should be more aligned with HYGOV/REGCA/ESDC1A.

I learned a lot as I developed these, so I am not surprised that each one has diverged so much. Thank you for the detailed review I value the standardization as well.

and Rebased

[nkoukpaizan]

A few minor comments. I'll merge when these are addressed and the branch is rebased.

[lukelowry]

Rebased and addressed your final comments. I left one unresolved so we can make sure we are on the same page