In [`common_pop`, we need to initialize `fpu`, `lmf` and `cmi` explicitly](https://github.com/worlddynamics/WorldDynamics.jl/blob/6d01563db3c6187e4aa19923ec3a588fb27577e8/src/World3/population/common_pop/subsystems.jl#L13-L15) for the [plots in `pop15` ](https://github.com/worlddynamics/WorldDynamics.jl/blob/6d01563db3c6187e4aa19923ec3a588fb27577e8/src/World3/population/pop15/plots.jl) to work. Why is that the case?