Add pyGSTi to software list

[kevincyoung]

pyGSTi is a software framework from Sandia National Laboratories' Quantum Performance Lab for the modeling, characterization and benchmarking of noisy quantum computers. It contains implementations of gate set tomography, randomized benchmarking, and many other characterization protocols. It contains a number of routines for simulating noisy and noiseless quantum processors, and can compute a wide range of performance metrics. It is capable of outputting code that can be directly run by several open-access quantum testbeds, and is widely used in the community.

I suggest adding the following to the full-stack software libraries:

---

  - description: 'Software framework for modeling, characterizing, and benchmarking noisy
      quantum computers developed by [Sandia''s Quantum Performance 
      Laboratory](https://qpl.sandia.gov/)'
    name: pyGSTi
    url: http://www.pygsti.info/