Incorrect atttribute of Crossspectrum used to calculate time lag in Simulator.ipynb tutorial

[bjricketts]

On line 600, the .cs attribute is used instead of .power to calculate the time lag. e.g. It should be: lag = np.angle(cross.power)/ (2 * np.pi * cross.freq) instead of: lag = np.angle(cross.cs)/ (2 * np.pi * cross.freq) as it currently is.

[matteobachetti]

@bjrickets thank you so much for catching this!