Bioinformatics Research Centre, Department of Computer Science and MADALGO, Center for Massive Data Algorithms, a Center of the Danish National Research Foundation, Aarhus University, DK-8000 Aarhus C, DenmarkBioinformatics Research Centre, Department of Computer Science and MADALGO, Center for Massive Data Algorithms, a Center of the Danish National Research Foundation, Aarhus University, DK-8000 Aarhus C, Denmark.