Get scenario site ids
Arguments
- init_subset_nwss_data
a dataframe of the raw NWSS data filtered to exclude solids and upstream sites
- scenario
a string indicating what scenario we are running. Default is "status_quo" which uses all the data we have available
- scenario_dir
a string indicating the file path where the scenario csvs will live. Default is NA because we don't need this for the status_quo scenario