calibrate_observable_estimates mutates PauliTerm instead of using interface

[jlapeyre]

This code mutates the coefficient attribute of PauliTerm directly

forest-benchmarking/forest/benchmarking/observable_estimation.py

Lines 1001 to 1003 in 61a468f

	observables = [copy(res.setting.observable) for res in expt_results]
	for obs in observables:
	obs.coefficient = complex(1.)

But, PauliTerm offers a an interface that does the same thing

https://github.com/rigetti/pyquil/blob/d45246d183a7308187bc28442c73714e87f7c25a/pyquil/paulis.py#L557-L570