- as of September 2021, PyCharm is used to write some of the code
- PyCharm's built-in code styler, linter, and type checker issued
some warnings that are resolved in this commit
+ spelling mistakes
+ all instance attributes must be specified explicitly
in a class's __init__() method
=> use `functools.cached_property` for caching
+ make `tuple`s explicit with `(...)`
+ one test failed randomly although everything is ok
=> adjust the fixture's return value (stub for Google Directions API)
+ reformulate SQL so that PyCharm can understand the symbols
- move the module
- unify the corresponding tests in `tests.forecasts.methods` sub-package
- make all `predict()` and the `stl()` function round results
- streamline documentation
- add `.low80`, `.high80`, `.low95`, and `.high95` columns
- add check contraints for the confidence intervals
- rename the `.method` column into `.model` for consistency
