Supporting successful implementation of public health interventions: protocol for a realist synthesis by MacDonald, M, Pauly, B, Wong, G, Schick-Makaroff, K, van Roode, T, Wilson-Strosher, H, Kothari, A, Valaitis, R, Manson, H, O'Briain, W, Carroll, S, Lee, V, Tong, S, Dickerson-Smith, K, Ward, M