Reduced version of the model DIETER

This is a much-reduced version of the model DIETER (Dispatch and Investment Evaluation Tool with Endogenous Renewables). This version is used for the stylized graphical illustrations in the review paper "The Economics of Variable Renewables and Electricity Storage", written by Javier Lopez Prol and Wolf-Peter Schill. This update 1.2.1 features some minor improvements compared to the previous version 1.2 as regards reportig and model runs where VRE expansion is driven by CO2 pricing, and not by a constraint. Previous versions were used for the Commentary "Electricity storage and the renewable energy transition", https://doi.org/10.1016/j.joule.2020.07.022, written by Wolf-Peter Schill, and for the paper "On the economics of electrical storage for variable renewable energy sources", https://doi.org/10.1016/j.euroecorev.2018.07.004, written by Alexander Zerrahn, Wolf-Peter Schill and Claudia Kemfert. Whenever you use this code, please refer to these papers and/or the sources listed under http://www.diw.de/dieter