Fix demand cloning bug

While demand cloning was happening for the `sample` attribute,
the units were left unchanged, leading to missing values down the
line. It only took two lines of code to fix this, using the existing
reindexing operation followed by normalizing the index of `units`.

pelicun/model/demand_model.py

@@ -859,6 +859,9 @@ def turn_to_tuples(demand_cloning):
         self.sample = self.sample.iloc[:, column_index]
         # update the column index
         self.sample.columns = pd.MultiIndex.from_tuples(column_values)
+        # update units
+        self.units = self.units.iloc[column_index]
+        self.units.index = self.sample.columns
 
     def generate_sample(self, config):
         """