Extract base conversion functions

[lukelowry]

Description

Consolidates PhasorDynamics component power-base conversion and consistently requires explicit component ratings instead of falling back to the system base.

Proposed changes

• Add shared component-base storage and conversion helpers to Component.
• Use the shared conversion across the affected machines, governors, converter, and controllers.
• Require mva for REECB and Trate for GASTPTI and HYGOV.
• Remove duplicated model-specific conversion helpers and storage.
• Update the corresponding documentation and tests.

Checklist

• All tests pass.
• 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 are N/A; this is a focused cleanup of existing model implementations.

Further comments

None