sktime STLTransformer is not detrending the series, current implementation is only deseasonalising

[ltsaprounis]

In the notebooks we treat the STLTransformer as a detrender / deseasonaliser. However the sktime implementation is only a deseasonaliser. As a quickfix I'll add the sktime detrender as a step in all forecasting pipelines with STL. Will also raise an issue in sktime to fix this.