Functions output is inconsistent (different number of columns) when querying one/multiple sites.

[cjbas22]

Issue

The following functions: get_qwdata, get_discharge_measurements, get_discharge_peaks, get_gwlevels, get_stats, get_dv, get_info, get_iv will output a dataframe with a datetime index when a single site is provided and a dataframe with a MultiIndex (site/datetime) when more than 1 site is provided.

To Reproduce

Single site:
site_id = "434400121275801"
nwis.get_gwlevels(sites=site_id)

More than one site:
site_ids = ["434400121275801", "375907091432201"]
nwis.get_gwlevels(sites=site_ids)

Expected behavior
For consistency, both queries should return similar dataframes.

[sblack-usu]

There are two more scenarios:

nwis.get_gwlevels(sites=site_id, multi_index=True) # what happens here?  Can a single site produce a multi_index?

nwis.get_gwlevels(sites=site_ids, multi_index=True) # This will produce the previous versions result.

[cjbas22]

If a single site is provided the output is always a dataframe with a single level (datetime) Index. multi_index doesn't do anythign in this case as format_response checks "if len(df['site_no'].unique()) > 1" first.