Nat. Methods 14, 23–31 (2017); published online 29 December 2017; corrected after print 10 February 2017

In the version of this article initially published, author affiliation numbers were incorrect. Xiaoyu Li originally had affiliation 1; this has been changed to affiliations 1 and 2. Xushen Xiong originally had affiliations 1 and 2; these have been changed to affiliations 1–3. Chengqi Yi originally had affiliations 1 and 3; these have been changed to affiliations 2 and 4. The error has been corrected in the HTML and PDF versions of the article.