Integrate network latency

[stsievert]

It'd be nice to have the latency for different networks to see how much that influences the results. Currently, the network object only looks at bandwidth: https://github.com/TalwalkarLab/paleo/blob/984bbf5d2942f28b8599db4374c0ad788efe0f6e/paleo/device.py#L38-L54