GAST-PTI Documentation + Implementation

[lukelowry]

Description

Implementation and documentation for the GASTPTI PhasorDynamics governor model.

Proposed changes

• Added GASTPTI model implementation, documentation, and diagram.
• Added GASTPTI JSON parsing and SystemModel assembly support.
• Added focused PhasorDynamicsGovernorGastPtiTest unit coverage.
• Updated governor 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

The initialization of Pmech in relation to the machine is not ideal and still relies on the machine being initialized first. It makes reading the implementation confusing, in my opinion. I am partial to changing how we initialize models like this (see #405)