1. Academy of Mathematics and Systems Science, Chinese Academy of SciencesBeijing 100080, People's Republic of China
2. Department of Biology, York UniversityToronto, Ontario M3J 1P3, Canada
3. Department of Computer Science and Engineering, York UniversityToronto, Ontario M3J 1P3, Canada
4. Department of Mathematics and Statistics, York UniversityToronto, Ontario M3J 1P3, Canada