Health outcomes, pathogenesis and epidemiology of severe acute malnutrition (HOPE-SAM): rationale and methods of a longitudinal observational study by Bwakura-Dangarembizi, M, Amadi, B, Bourke, C, Robertson, R, Mwapenya, B, Chandwe, K, Kapoma, C, Cifunda, K, Majo, F, Ngosa, D, Chakra, P, Chuku, N, Masimba, F, Mapurisa, I, Besa, E, Mutasa, K, Mwakamui, S, Runodamoto, T, Humphrey, J, Ntozini, R, Wells, J, Manges, A, Swann, J, Walker, A, Nathoo, K, Kelly, P, Prendergast, A