Function to show when PT or PT + QEM is helpful

[purva-thakre]

We want to add a function in mitiq/pt that spits out a graph showing when PT is useful for some circuit and when combining PT with some QEM technique is helpful.

compare_performance(
  circuit, 
  noisy_backend with coherent noise, 
  num_pauli_twirled_circuits_to_create, 
  {metadata of some QEM if combining PT with QEM}
)

This should spit out a plot showing when combining noise tailoring with QEM techniques is useful as a function of noise strength.

See the example plot below where the difference between ideal and other expectation values are plotted. The closer the curve is to 0.0, the better the technique performs. It is also easy to see the regions when ZNE is useful on its own, PT is useful on its own, combination of PT + ZNE is useful, etc.