Computing expectation values from frequencies

[bpcarlos]

I believe it will be useful to have a class in Qibo to compute expectation values of sZ operators directly from output frequencies. This might be useful for simulations with sampling, and implementations in real hardware.

Ideally, I would like to give the output frequencies and the qubits involved in the operator and obtain the expectation value. Say, I want to compute the expectation value of sZ(0)*sZ(2)*sZ(3). Then, I would deliver the output frequencies and select the qubits 0, 2 and 3, and the expectation value should be directly computed by simply reading the probabilities of measuring 0 and 1 on those qubits.

[bpcarlos]

Actually, this is related to #366