An open letter to The BMJ editors on qualitative research - Greenhalgh, T; Annandale, E; Ashcroft, R; Barlow, J; Black, N; Bleakley, A; Boaden, R; Braithwaite, J; Britten, N; Carnevale, F; Checkland, K; Cheek, J; et al. (BMJ, 2016)