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)