MVA Parameter Cleanup

[lukelowry]

Description

Use mva consistently as the JSON-facing key for synchronous machine MVA base parameters.

Proposed changes

• Replaces synchronous machine mva_base JSON keys with mva.
• Updates synchronous machine data enums, tests, and example cases to use the new key.
• Updates the PhasorDynamics input format documentation for the renamed parameter.

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.

Further comments

More consistent style, but this is also a cleanup in preparation for a more general power base conversion API, which needs to be consistent as we introduce more intra-model initialization (especially for REECA, REECB, REPCA, and HYGOV)