Measure expectation value of Hamiltonian

[kevinsung]

trafficstars

It should be optimized for the tradeoff between the number of measurements and the variance of the result (e.g. section IV.A of https://arxiv.org/pdf/1801.03524.pdf and other things like grouping together terms of the Hamiltonian). @ncrubin probably has already implemented some of this in a different context.

Blocked by #1299

[kevinsung]

#2781 is related to this.

[tanujkhattar]

We have PauliSumCollector to estimate expectation value of a hamiltonian represented as a linear combination of pauli observables. We also have measure_observables and measure_grouped_settings to measure a collection of (pauli string) observables for a state prepared by a given circuit. I'll go ahead and close this issue but please feel free to reopen if you think these utilities are not sufficient for your use case.