European guidelines for quality assurance in colorectal cancer screening and diagnosis: Overview and introduction to the full Supplement publication by Von Karsa, L, Patnick, J, Segnan, N, Atkin, W, Halloran, S, Lansdorp-Vogelaar, I, Malila, N, Minozzi, S, Moss, S, Quirke, P, Steele, R, Vieth, M, Aabakken, L, Altenhofen, L, Ancelle-Park, R, Antoljak, N, Anttila, A, Armaroli, P, Arrossi, S, Austoker, J, Banzi, R, Bellisario, C, Blom, J, Brenner, H, Bretthauer, M