The Norwegian Cognitive impairment after stroke study (Nor-COAST): study protocol of a multicentre, prospective cohort study by Thingstad, P, Askim, T, Beyer, MK, Bråthen, G, Ellekjær, H, Ihle-Hansen, H, Knapskog, AB, Lydersen, S, Munthe-Kaas, R, Næss, H, Pendlebury, ST, Seljeseth, YM, Saltvedt, I