Text and Data Mining valid from 2018-10-29
Received: 13 March 2018
Accepted: 21 September 2018
First Online: 29 October 2018
Change Date: 7 November 2018
Change Type: Correction
Change Details: In the version of this Letter originally published online, the ORCID number, 0000-0002-8900-3237, of the author A. Josten was missing; and in Fig. 2b, in the <i>y</i> axis label, ‘×10<sup>5</sup>’should have been ‘×10<sup>3</sup>’. These errors have now been corrected in all versions.
: The authors declare no competing interests.