Nat. Methods 11, 183–189 (2014); published online 8 December 2013; corrected after print 5 February 2014

In the version of this article initially published, the current affiliation of author Ralph Sperling was not included. His current affiliation is the Fraunhofer ICT-IMM, Mainz, Germany. The error has been corrected in the HTML and PDF versions of the article.