GENSAL Machine Model

[lukelowry]

Description

Implementation and documentation for GENSAL model.

Proposed changes

• Saturation model adjustments
• GENSAL Documentation
• GENSAL Implementation
• GENSAL Tests

Checklist

• All tests pass (except that one known error in PowerElectronics)
• 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 consistency improvements with GENROU and parameter edge cases

[abirchfield]

This is good to do. It would be better if there were a 2-bus case matching PowerWorld to make sure it's right.

[lukelowry]

This is good to do. It would be better if there were a 2-bus case matching PowerWorld to make sure it's right.

I will add that to this PR because I agree here