Received: 6 October 2017
Accepted: 19 June 2018
First Online: 22 August 2018
Ethics approval and consent to participate
: All participants signed written informed consent forms. The study protocol was considered by the Regional Committee for Medical and Health Research Ethics in Norway (REC) (No. 2015/1299), with the conclusion that the project did not require approval from REC. The study protocol was approved by the Norwegian Centre for Research Data (NSD) (No. 44755).
: Not applicable.
: The authors declare that they have no competing interests.
: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.