need sim identifier if slim = FALSE

In `qi_builder` if slim = FALSE and we get returns of simulated values, can we also get a column stating which sim a value came from? This would 1) make it easier to use the `group` argument in a ggplot to, say, make a bunch of densities of predicted values to compare to the distribution of the data and 2) mean that we could tell more easily when different simulated predictions came from different initial data points (e.g., if X1 = X2, if they're from the same simulation, we know they are different data points).