Only allow users to get "predicted_counts" and "global_rt" if not fitting to wastewater data

[kaitejohnson]

Goal

Currently, if the user switches off include_ww = 0 they still can request a dataframe for subpop_rt and predicted_ww. These will be based on the priors/may not output in future versions, and so we should under the hood only allow them to get two of the four possible outputs here

Context

#153 (comment)