Option in neutralize_variable to neutralize children

[MaxGhenis]

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.