feat: implement gravity on current piece

Tetris.py

@@ -40,9 +40,11 @@ class Tetris:
     # gets called from the games main loop
     def update(self) -> None:
         # TODO write not initialized exception
+        elapsed_time = self.clock.tick(self.fps)
+
+        if self.current_piece:
+            self.current_piece.update(elapsed_time, self.well, self.stack)
 
-        self.clock.tick(self.fps)
-         
         # TODO create control utility class
         for event in pygame.event.get():
             if event.type == pygame.QUIT: