OpenFermion-Cirq v0.3.0

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