cs215/unit04/prob01_actor_centrality.py added, but too slow
cs215/unit04/prob01_actor_centrality.py: optimized, specifically for pypy
cs215/unit04/prob01_actor_centrality.py: use deque
cs215/unit04/prob06_actor_centrality.py renamed
cs215/unit04/prob02_minimize_abs_value.py added
cs215/unit04/prob03_minimize_sum_squares.py added
cs215/unit04/prob04_mode.py added
cs215/unit04/prob05_up_heapify.py added