Use system base for governor-machine power signals

[lukelowry]

Description

Use system-base values for governor-machine mechanical power signals.

This is needed because governor ratings are not generally the same as the associated machine MVA base. Governor pmech signals, or equivalent governor-machine signals, should therefore specify their power base (Turbine rating in this context). This PR introduces Trate as the TGOV1 component base and converts at the governor signal boundary.

Proposed changes

• Add Trate as the TGOV1 component-base parameter.
• Convert TGOV1 mechanical power between system base and TGOV1 component base.
• Convert GENROU and GENSAL pmech signal interaction to use system-base values.
• Update example cases so TGOV1 Trate matches the associated machine mva.
• Update TGOV1 tests and tiny example inputs for the new base semantics.

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

No functional change. This is just a component contract change that I need to establish before I add more complex governors

I made this change as small as possible to fix the TGOV1 issue. Long term a more general base conversion might be prefered but this by no means locks us into a convention.