Deduplicate expectation value code in simulators

[95-martin-orion]

After PR #3979, we will have separate but near-identical implementations of simulate_expectation_values_sweep defined in sparse_simulator and density_matrix_simulator. it should be possible to deduplicate this code by moving it into a mutual parent class of these simulators - either SimulatorBase or a new subclass of it.

[mhucka]

Belatedly adding the triage/accepted label, after reviewing and thinking about the desirability of follow through on this. (Which is: low priority, but still important enough to be worth keeping the issue open.)