ARIMA with linear regressors time series modelling

This was mentioned in some other issues that have now been closed following the first release of time series functionality.

We currently use ARIMA with linear regressors. There may be better algorithms but it works well for us and is fairly simple to understand and implement.

If you can add basic models like this and somebody produces a C#/F# equivalent of the [Forecasting Principles and Practices e-book](https://otexts.com/fpp2/intro.html) then you might be able to support quite a few people who are just getting started with time series forecasting.

Beginne damit, die vorhandene Zeitreihenfunktionalität des Repositorys und die in der Beschreibung referenzierten geschlossenen Issues zu prüfen. Verwende den Anwendungsfall ARIMA-with-linear-regressors und das verlinkte E-Book Forecasting Principles and Practices, um den Umfang mit den Maintainer:innen zu klären. Als abgeschlossen gilt ein definiertes Basismodell mit entsprechender C#/F#-Unterstützung.

Vom Indexierungsmodell aus dem Issue-Text verfasst.