defined_for doesn't work for simulation-defining formulas

[nikhilwoodruff]

This is a pretty complex edge case I didn't consider when writing the defined_for logic. The way that defined_for works is by intercepting the entity(variable, period) calls inside a subsetted variable's formula and pre-subsetting them, so normal operations on them return the subsetted population results. But no interception happens when a formula creates a new simulation and uses outputs from simulation.calculate.