Openfermioncirq

Notable changes and additions:
- An implementation of the fermionic Fast Fourier Transform: `openfermioncirq.ffft`.
- Improved Givens rotation network for effecting one-body basis rotations: `openfermioncirq.primitives.optimal_givens_decomposition`.
- `openfermioncirq.CubicFermionicSimulationGate`.
- `openfermioncirq.QuarticFermionicSimulationGate`.

Deprecations:
- `openfermioncirq.XXYYPowGate`. Use `cirq.ISwapPowGate` with negated exponent, instead.
- `openfermioncirq.XXYY`. Use cirq.ISWAP with negated exponent, instead.
- `openfermioncirq.YXXYPowGate`. Use `cirq.PhasedISwapPowGate`, instead.
- `openfermioncirq.YXXY`. Use `cirq.PhasedISwapPowGate`, instead.
- `openfermioncirq.CXXYYPowGate`. Use `cirq.ControlledGate` and `cirq.ISwapPowGate` with negated exponent, instead.
- `openfermioncirq.CXXYY`. Use `cirq.ControlledGate` and `cirq.ISWAP` with negated exponent, instead.
- `openfermioncirq.CYXXYPowGate`. Use `cirq.ControlledGate` and `cirq.PhasedISwapPowGate`, instead.
- `openfermioncirq.CYXXY`. Use `cirq.ControlledGate` and `cirq.PhasedISwapPowGate`, instead.

Notable changes and additions:
- `CombinedDoubleExcitationGate`
- `bogoliubov_transform` and `prepare_gaussian_state` automatically detect spin symmetry and return optimized circuits if it is present
- Implemented FSWAP as an EigenGate, so it can be raised to a power
- Standardized common gate classes to take an `exponent` argument and have a name of the form `NamePowGate`. For example, `XXYYGate` is now `XXYYPowGate` and no longer takes rads, degs, or half_turns.
- Faster simulation of most gates on Cirq's simulator via defining the `_apply_unitary_to_tensor_` magic method
- `rot11`, `rotzz`, `Rxxyy`, `Ryxxy`

Includes major changes to functionality for studying variational algorithms and the low rank Trotter step.
- Changed the protocol for defining a variational ansatz
- The objective function for variational algorithms is now represented by the VariationalObjective class
- VariationalStudy can track additional information, such as time spent, about optimization runs
- Utilize improved low rank decomposition from OpenFermion for low rank Trotter step
- Changed how the one-body terms are simulated in the low rank Trotter step
- Added a variational ansatz based on the low rank Trotter step

Minor package polish.

- Now requiring python 3.5.3+ instead of 3.5+ (https://github.com/quantumlib/Cirq/issues/710).
- Fixed error in variational algorithm tutorial.
- Put user installation instructions before dev instructions.
- Various doc fixes.

The first public alpha release of OpenFermion-Cirq! Launch features include:

* Linear depth linear connectivity Bogoliubov transformations.
* Framework for linear depth linear connectivity swap networks.
* Framework for implementing different types of Trotter steps
* Linear swap network DiagonalCoulombHamiltonian Trotter steps.
* Linear split operator DiagonalCoulombHamiltonian Trotter steps.
* Prototype functionality for arbitrary "low rank" Trotter steps.
* Framework for instantiating variational ansatz circuits.
* Framework for implementing variational algorithm studies.
* Variational ansatz circuits based on linear Trotter steps.
* Tutorial about basis transformations and free fermion models.
* Tutorial about Trotter steps of DiagonalCoulombHamiltonians.
* Tutorial about Trotter steps based on low rank decompositions.
* Tutorial about implementing variational algorithms, applied to jellium, H2.