Protocol for the SELECT study: a sequential mixed methods study of the selection of UK medical students into clinical academic training by Byrne, MHV, Chan, C, Karas, A, Lynn, E, Dominic, C, Bain, R, Wan, JCM, Clelland, AD, Hayes, S, Asif, A, Harvey Bluemel, A, Mogg, J, Lawrence, L, Church, H, Finn, G, Brown, MEL