Hubbard model Trotter steps

[babbush]

Eventually it would be great to implement the sqrt(N) depth Trotter steps described in arXiv:1711.05395 - perhaps @john6060 would be interested in working on this?

But more immediately I think it would be really helpful for people to see some examples of using OpenFermion-Cirq for Hubbard Trotter steps. Because the algorithm of arXiv:1711.04789 is much closer to the linear swap networks we've already implemented, I think we should try to add that in the next couple weeks, perhaps with a small tutorial. @kevinsung would you want to do this? How much would it require beyond what we already have?

[Strilanc]

If we are going to add more and more examples, I think we need to come up with some sort of strategy for running the tests that doesn't add multiple minutes to every build. I'll think more about it.

[babbush]

I'd say that we're probably at about 60% of the tutorials I'd like to have once we add some rather basic functionality over the next few months.

[kevinsung]

@Strilanc most of the extra build time from the tutorials is from the simulator, so the first thing is to upgrade the simulator.

[Strilanc]

That's true, the coming simulator improvements will help a lot.