Support zero IEEET1 transducer time constant

[lukelowry]

Description

Support Tr = 0 in the IEEET1 exciter by treating the sensed terminal voltage state as algebraic when the transducer time constant is zero.

This removes the need for small artificial Tr values in validation and example cases.

Proposed changes

• Tag IEEET1 Vts as algebraic when Tr = 0.
• Update the IEEET1 README equations to match the implementation and standard notation
• Update IEEET1 example, validation, and parser-test data so prior tiny Tr placeholders are zero.
• Add unit-test for the Tr = 0 algebraic behavior.

Checklist

• All tests pass for the focused coverage listed below.
• 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.

Changelog changes N/A

Further comments

Early development known issue that did not matter until detailed validation of the larger models.

[nkoukpaizan]

There are potential issues with comparing a floating point value to 0 like this.