Move decomposition module into methods sub-package

- move the module
- unify the corresponding tests in `tests.forecasts.methods` sub-package
- make all `predict()` and the `stl()` function round results
- streamline documentation

src/urban_meal_delivery/forecasts/__init__.py

@@ -1,5 +1,4 @@
 """Demand forecasting utilities."""
 
-from urban_meal_delivery.forecasts import decomposition
 from urban_meal_delivery.forecasts import methods
 from urban_meal_delivery.forecasts import timify