1. Center for Computational Biology, Flatiron Institute, New York, NY 10010 USA.
2. Center for Computational Mathematics, Flatiron Institute, New York, NY 10010 USA.
3. Dipartimento di Matematica, Università di Pisa, 56127 Pisa, Italy.
4. Center for Computational Biology, Flatiron Institute, New York, NY 10010 USA, Department of Molecular Biology, Princeton University, Princeton, NJ 08540 USA, and Lewis-Sigler Institute for Integrative Genomics, Princeton University, Princeton, NJ 08540 USA.
5. Corresponding author. Dipartimento di Matematica, Università di Pisa, 56127 Pisa, Italy, and Department of Mathematical Sciences, New Jersey Institute of Technology, Newark, NJ 07102 USA.