Option in `neutralize_variable` to neutralize children

For example, suppose we want to model the repeal of all state tax exemptions. One shortcut could be creating a variable for state_exemptions which calls adds over all state-level exemption variables, then calling neutralize_variable -- but by default this wouldn't actually do anything, since it's just a helper variable. If we added an arg like neutralize_children, we could then use that to model repealing all those variables at once.

Start by locating neutralize_variable and the adds behavior in the Python core engine, then inspect existing tests for variable neutralization. Define how neutralize_children should affect helper variables and verify the requested repeal use case with focused tests. Done means the option works without changing default neutralization behavior.