Leakage Randomized Benchmarking

[ShellyGarion]

What is the expected behavior?

Characterization of leakage errors according to:
Christopher J. Wood, Jay M. Gambetta, Quantification and Characterization of Leakage Errors, https://arxiv.org/abs/1704.03081

The RB sequences are generated using standard RB, so we need to add to the RB code:

• A new fitter class LeakageRBFitter to fitters.py
  we fit the standard RB output to the decay model A+Bp^m
  and compute the estimate of the average leakage and seepage rates of the gate set as
  L1 = (1-A)(1-p)
  L2 = A(1-p)

• Test for the fitter and a function to generate test data using the pulse simulator