Surgical capacity, productivity and efficiency at the district level in Sub-Saharan Africa: A three-country study by Zhang, M, Gajewski, J, Pittalis, C, Shrime, M, Broekhuizen, H, Ifeanyichi, M, Clarke, M, Borgstein, E, Lavy, C, Drury, G, Juma, A, Mkandawire, N, Mwapasa, G, Kachimba, J, Mbambiko, M, Chilonga, K, Bijlmakers, L, Brugha, R