Simulation parameter to freeze state income taxes when modeling federal tax reforms

This isn't relevant yet since we freeze all state income taxes when computing nationwide impacts, but we plan to change that when we complete all state income tax models. Many state tax policies model after federal policies, so when we do this, the default behavior would be for state income taxes to change under certain federal tax reforms. While we would separate the budgetary impacts between federal and state taxes, distributional and cliff impacts would include both. Some users will want to assume that states respond by tying their laws to pre-reform federal laws.

The issue names no files, tests, or entry points. Begin by reviewing how state income taxes and federal tax reforms are modeled, then define the simulation parameter's behavior for states tied to pre-reform federal laws and how budgetary, distributional, and cliff impacts should be reported.

Written by the indexing model from the issue text.