Key traveller groups of relevance to spatial malaria transmission: a survey of movement patterns in four sub-Saharan African countries by Marshall, JM, Touré, M, Ouédraogo, AL, Ndhlovu, M, Kiware, SS, Rezai, A, Nkhama, E, Griffin, JT, Hollingsworth, TD, Doumbia, S, Govella, NJ, Ferguson, NM, Ghani, AC