Received: 28 November 2017
Accepted: 21 May 2018
First Online: 9 August 2018
Ethics approval and consent to participate
: The study protocol was approved by the Heinrich-Heine University (Düsseldorf, Germany) ethics committee.
: Ausra Ramanauskaite, Kathrin Becker, Gintaras Juodzbalys, and Frank Schwarz declare that they have no competing interests.
: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.