Rename Series output to 'mean' in analysis utils (#1571)

Previously, only DataFrame outputs had their columns renamed to 'mean.' This update ensures consistency by renaming Series outputs to 'mean' as well. It improves clarity and aligns with the expected nomenclature.

src/tlo/analysis/utils.py

@@ -471,6 +471,9 @@ def summarize(
     if isinstance(output, pd.DataFrame):
         output = output.rename(columns={'central': 'mean'},
                                level=0 if output.columns.nlevels == 1 else 1)
+    else:
+        output.name = 'mean'  # rename the series to mean
+
     return output