Distinction of Magnetization vs. Line Shunt (Validation)

[lukelowry]

Description

Implementation on develop treats the shunt as a magnetizing admittance, which should have a distinct parameter from the line shunt admittance, which does not observe the effects of phase/tap changes.

Before these changes, the Branch model is presumed to be either a XFMR or a line but not a line & XFMR in series. We need to support this edge case

MATPOWER does not seem to make this distinction, but PowerWorld models this correctly. This contributed marginally to errors in magnetization admittances in some cases.

Required before 0.2 release.

Proposed changes

• Add magnetic charging admittance term
• Move G and B contribution outside of tap/phase transformation

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.
• NA 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.

Further comments

Need to update the circuit mdoel diagram in future

[lukelowry]

An updated circuit diagram is outside the scope of this PR, so marking ready. The proposed changes also help future proof our Branch model so it can be used for XFMR, line, and composite XFMR-line model.