Allow a simulation to be updated after a change to the simulation script

There may be cases in which an advanced user wants to change update a simulation (via `update_sim`) after a change to the simulation script. One example of this is fixing a bug in the simulation script that only affects a subset of level combinations but does not affect replicates that have already run. This could look like this:

```R
sim %<>% update_sim(allow_script_change = TRUE)
```