Readout correction in operator estimation

[mpharrigan]

following #682

Provide faculties for automatic readout correction. Probably depends a bit on #712

A simple correction scheme where for each Experiment you measure your ansatz in a specified basis and the identity in the same basis. Use the second measurement to re-normalize the first.

cc @msohaibalam

[msohaibalam]

cc @ryankarle

[marcusps]

I am not sure what you mean by ansatz and identity here, @mpharrigan. That aside, I think the only way to make this scale in a sane way is to depend on symmetrization ( #712 ). With that, assuming readout error correlations are negligible, you just need to calibrate N parameters for as many qubits, and rescale observed expectations based on which qubits have a non-trivial Pauli operator (for the expectation value).