Nature Communications 5: Article number: 3756 (2014); Published: 29 April 2014; Updated: 26 June 2014; Updated: 10 October 2014.

In the original version of this Article, the middle initial of the author A.K. Walker was incorrectly given as L. This has now been corrected in both the PDF and HTML versions of the Article, and also the prior Corrigendum Nat. Commun. 5, 4264 (2014).