Cirq v0.13 release

[Orion Martin]

Dear Cirq Users,

We have just released Cirq minor version v0.13.0.

Highlights of this release include:

1. ProjectorString + ProjectorSum

Added cirq.ProjectorString and cirq.ProjectorSum for compact representations and calculations involving projection operations.

2. Gatesets

Construct cirq.GateFamily and cirq.Gatesets to test gates and operations for membership in well-defined groups.

3. NamedTopologies

Basic circuit to device placement routines (get_placements, draw_placements) have been added to Cirq, powered by NamedTopology.

4. Simulator performance improvements

Cirq's built-in simulators now dynamically factorize the simulated state into non-entangled subsets of qubits (when possible). This dramatically improves performance for circuits consisting of disconnected subsystems, and permits simulation of larger numbers of qubits for such circuits.

5. Clifford Tableau Decomposition

Upgraded existing clifford operations and circuits to make use of Tableau representation where appropriate and implemented baseline tableau to circuit decomposition using methods outlined in this paper.

You can view a complete list of features, fixes and breaks for this release here.

To install this upgrade run `pip install cirq==0.13.0`

Thank you to all our contributors for this release:

Adam Zalcman, Alapan Chaudhuri, Ali Panahi, Ana Sofia Uzsoy, Animesh Sinha, Antoine (Tony) Bruguier, Balint Pato, Bicheng Ying, cognigami, Dave Bacon, Dax Fohl, Doug Strain, Eric Hulburd, Guen Prawiroatmodjo, ishmum123, Ishmum Jawad Khan, Mark Daoust, Matthew Harrigan, Matthew Neeley, melonwater211, MichaelBroughton, Nathanael Thompson, Orion Martin, Pieter Eendebak, Ryan LaRose, Shrill Shrestha, Smit Sanghavi, Tanuj Khattar, twojno, Victory Omole, Zeeshan Ahmed

Happy Cirq-ing,

Orion