Implement a good way of tracking measurements

[nelimee]

Measurements need to be translated to record offsets in order to be readable and usable by Stim. This is currently handled by CircuitMeasurementMap that takes a cirq.Circuit instance as input and provides a way to compute the record offset for any measurement in this cirq.Circuit instance.

The current implementation of CircuitMeasurementMap is not adapted to the movement of a logical qubit due to one "limitation" (that was originally a feature): it counts measurements in a repeated cirq.CircuitOperation only once.

This means that if we perform the following QEC experiment:

1. Initialize a logical qubit,
2. Extend the logical qubit horozontally (let's say to the right),
3. Wait a few (let's say $n_1$) QEC cycles,
4. Measure out the data qubits on the left, where the logical qubit originally was,
5. Wait a few (let's say $n_2$) QEC cycles,
6. Measure the final observable,

the CircuitMeasurementMap would not be able to accurately compute the measurement records of the measurements included in the final observable, because it would count each measurement in the $n_2$ QEC cycles only once (instead of $n_2$ times).

A quick fix would be to change the behaviour of CircuitMeasurementMap on cirq.CircuitOperation instances when they are repeated.

[nelimee]

Thinking about it a little bit more, the CircuitMeasurementMap and all the code that is based on the assumption that a cirq.CircuitOperation only accounts for 1 cirq.Moment should probably be changed.

One instance of cirq.CircuitOperation can (and very likely will) take more than one cirq.Moment, and scheduling the measurements correctly in all cases with that fact is a little bit harder than what was expected.

For the moment, I think that the best would be to (potentially recursively) flatten the cirq.Circuit instance provided to the CircuitMeasurementMap constructor, construct the measurement map from that flattened circuit, and adapt the measurement indices provided to CircuitMeasurementMap.get_measurement_relative_offset.

[nelimee]

Fixed by #55