Acknowledgements
This work was cofunded by the European Union (European Regional Development Fund – Investing in your Future) and the German Federal State North Rhine-Westphalia (PtJ-Az.: z0911bt020b). The authors would like to thank Monika Huben, Martina Praast and Sabine Wortmann for their excellent technical assistance.
Potential conflict of interest
Disclosure forms provided by the authors are available with the full text of this article online at https://doi.org/10.1080/10428194.2017.1330955.