[WIP] Added a comment with ideas on how I could optimize day 14

src/day14.rs

@@ -73,6 +73,13 @@ impl Day14 {
     }
 
     fn simulate_sand(&self, grid: &mut Grid<Cell>) -> SimulationResult {
+
+        // TODO test optimization ideas:
+        // 1. Store a stack of the path the sand took. When executing the next one, go up the stack and use the position
+        //    that is found as the starting point for the current grain of sand
+        // 2. Just store the previous position of the sand before it came to rest, and start the siulation from there.
+        // 3. Use an quadtree for a represenation of the sand
+
         let grid_height = grid.height();
         let mut sand_coord = STARTING_COORD.clone();
             loop {