Add Literature section

tex/2_lit/2_class/1_intro.tex

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+\subsection{Demand Forecasting with Classical Forecasting Methods}
+\label{class_methods}
+
+Forecasting became a formal discipline starting in the 1950s and has its
+    origins in the broader field of statistics.
+\cite{hyndman2018} provide a thorough overview of the concepts and methods
+    established, and \cite{ord2017} indicate business-related applications
+    such as demand forecasting.
+These "classical" forecasting methods share the characteristic that they are
+    trained over the entire $Y$ first.
+Then, for prediction, the forecaster specifies the number of time steps for
+    which he wants to generate forecasts.
+That is different for ML models.