Add a library of standard device topologies

[karalekas]

In the device.py file, we have a set of classes that define a schema for a quantum computer. In the api/_quantum_computer.py file, we have a collection of private functions that allow users to generate QuantumComputer objects from Rigetti QPUs or with custom topologies. It would be great if it were really easy to choose from a collection of standard device topologies beyond just what is available on QCS and the 9q-square-qvm. Some examples include:

• the different variations of the Rigetti octagon-square tiling
• the 5Q T-shaped topology of ibmq_ourense and ibmq_vigo
• the 5Q +-shaped topology of ibmq_5_yorktown - ibmqx2
• IBM's newly-announced 53Q chip
• Google's 53Q Sycamore processor of recent fame