Issue 184: Add outputs to `generate_simulated_data()` fxn and package data

[kaitejohnson]

This PR closes #184. It adds the following outputs to the generate_simulated_data() function.

1. Wastewater data beyond the forecast period (called ww_data_eval)
2. Subpopulation level count data

Goal of including these outputs is to compare performance of forecasts at the subpopulation level (of both sets of observations).

Some open questions:

• should we include all of this as package data? Or is it fine just to have as outputs of generate_simulated_data()?
• the eval data as separate is probably redundant but it feels useful to have them as two different datasets otherwise we lose the fact that the data fit to the model in the vignette represents the bare minimum data requirements you need. I'm in favor of keeping them as separate datasets (since we want to encourage the use of vintaged data anyway), but curios others thoughts on this?

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[kaitejohnson]

Pending CI passes this is ready for re-review