SEXS-PTI Exciter Model + GENROU Saturation

[lukelowry]

Description

SEXS-PTI Model added; needed for some of the team's requested cases. Also, to validate the cases that depend on this PR, I need to add a temporary initialization routine for saturation. It is not how we should do this, but I simply need a steady-state solution to verify that the cases in question provide meaningful data or can be simulated at all.

Relies on the changes in the exciter and stabilizer input in #365, so PR is to that branch.

Proposed changes

• SEXS-PTI Exciter Model
• Temporary iteration in the initialization of GENROU to initialize nonlinear saturation for the steady state.

Checklist

• All tests pass. (There is One preexisting error that is happpening in the PowerElectronics module!)
• 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.

Further comments

Some of the equations regardining the smooth approximation might need to be updated.

This model is simple and widely used. Let's include this and its data format changes, so we don't need to change the cases as much.