Nat. Methods 7, 237–242 (2010); published online 14 Feburary 2010; corrected after print 29 September 2010.

In the version of this paper originally published, the name of and reference to the algorithm in the rightmost column of Table 1 were incorrect. The correct reference (ref. 40) has been added in the paper. The error has been corrected in the PDF and HTML versions of the article.