Add speed comparison for broadcasting exercise in NumPy lecture

[natashawatkins]

I saw that there was a new exercise in the NumPy lecture (Exercise 9.4), but I thought maybe it would be more useful for illustrating broadcasting if the reader is asked to use compare the speeds of broadcasting vs. using the for-loop they write.

[jstac]

Good point @natashawatkins .

@HumphreyYang , how about

• adding np.random.seed(123) here so the output is the same for all readers
• adding a second part to the exercise where 4 is replaced by a large number and the reader is asked to time a for loop against the broadcast implementation.