Add a SkiIQ drill

Start at 100, or at the skier's most recent average SkiIQ, or the average of his first 8 turns. Let the skier choose a different starting level if he prefers, as with the current drills. Move him up a level if he completes 16 out of 20 turns at level n+1. The smarts for calculating per-turn SkiIQ are already implemented, so all this would need is the drill scaffolding.