Generalize state preparation and measurement for noisy processes

[GTorlai]

Add the option to pass a POVM to getsamples, to generalize the Von Neumann measurement currently implemented. Also, we could consider allowing to pass a circuit (unitary or noisy, lazy or itensors) to getsamples which captures the rotation associated to a give local basis. For example, to measure in the Y basis one should apply Sdag and H, possibly with their own respective noisy channels. Instead, we now do an automated eigendecomposition of the measurement operator (which is still very handy).

The same reasoning applies for state preparation for generating process tomography data. There, we should allow single-qubit density operators to be passed as input states.

For all the above, we should also allow the user to pass a Choi matrix representation of a noisy gate in both state preparation and measurement rotations (instead a unitary tensor + a Kraus operators).