Create game versions of level_means_count and level_means_intersect and
move functionality of _count files to intersection_finder.m

- Create game_means_count and game_means_intersect which find mean
  scores for a specific game at all levels.
- Update level_means_count and level_means_intersect to remove game mean
  functionality (see above comment).
- Remove core functionality from _count files to intersection_finder.m.