River Rapids. Vortex. Twister. Stingray. If you know, you know. And if you don't, you weren't fortunate enough to be growing up in Edinburgh in the late 1980s through to the year 2000. I am referring of course to the legendary flumes of the city's Royal Commonwealth Pool. The flumes have been gone now for more than twice as long as they were with us, but for many they remain a bittersweet memory.…