Make cache_df decoration more apparent Klimatbyran#682

* Hint a return type that is the same as the decorated function.

data/issues/emissions/cache_utilities.py

@@ -5,7 +5,7 @@
 import pandas as pd
 
 
-def cache_df(f: type(pd.read_excel) = None, path: str = '', freq: str = '1Y'):
+def cache_df(f: type(pd.read_excel) = None, path: str = '', freq: str = '1Y') -> type(pd.read_excel):
     """
     Cache the DataFrame to an intermediate file and use it if created within the same period.