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

tests/forecasts/methods/__init__.py

@@ -0,0 +1 @@
+"""Tests for the `urban_meal_delivery.forecasts.methods` sub-package."""

tests/forecasts/methods/test_decomposition.py

@@ -7,7 +7,7 @@ import pytest
 
 from tests.forecasts.conftest import NS
 from tests.forecasts.conftest import VERTICAL_FREQUENCY
-from urban_meal_delivery.forecasts import decomposition
+from urban_meal_delivery.forecasts.methods import decomposition
 
 
 class TestInvalidArguments:

tests/forecasts/methods/test_ts_methods.py

@@ -1,4 +1,7 @@
-"""Test the `arima.predict()` and `ets.predict()` functions."""
+"""Test the `arima.predict()` and `ets.predict()` functions.
+
+We consider both "classical" time series prediction models.
+"""
 
 import datetime as dt