Return `Iterator` from simulator sweep methods

[95-martin-orion]

Simulator methods that return multiple results can be difficult to work with, since each result object may contains a massive representation of the simulator state. An example of this can be seen in #3979 and #3990, where the simulate_expectation_value_sweep method can't simply invoke the simulate_sweep method for fear of generating (and storing!) all of the output states simultaneously.

To resolve this, we should modify the simulator *_sweep methods to return Iterator objects instead of Lists.

This would also cause sweeps to run one simulation at a time, which reduces the "black box" delay of large sweeps, and may provide opportunities for parallelizing the simulation and result-processing pipelines.

[95-martin-orion]

Concern: how do we deprecate this while keeping the same name?

[95-martin-orion]

Concern: how do we deprecate this while keeping the same name?

Given the amount of time necessary (and confusion generated) from reusing the same names to return iterators, it's preferable to just have separate *_iter methods. The list-based methods can be thin wrappers around these.