Draft of new GST sample.

[cgranade]

This PR adds a new gateset tomography sample, complementing the existing process tomography and randomized benchmarking samples. In particular, this sample uses the pyGSTi package to infer noise models on a two-qubit {X, Y, CNOT} gateset.

Still in draft pending the following work items:

• [x] Add README
• [x] Link from binder-index.
• [x] Add short introduction to GST.
• [ ] Lock Q# version in environment.yml.