1. African Institute for Mathematical Sciences, 6 Melrose Road, Muizenberg, Cape Town, Republic of South Africa
2. Bernhard-Nocht Institute for Tropical Medicine, Bernhard-Nocht Straße, 74, Hamburg, Germany
3. Mathematical Institute, University of Oxford, Oxford OX2 6GG, UK
4. Department of Mathematics and Applied Mathematics, University of the Western Cape, Private Bag X17, Bellville 7535, Republic of South Africa