Support for Projectors in Expectation Calculations

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@MichaelBroughton is already working on this.

Since Sep 2, 2020.

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Description

kind/feature-request priority/review

Right now Cirq does not allow such things as:

a = cirq.Z(q0) * cirq.Z(q0) + projector(0111011001)
a.expectation_from_wavefunction(a, ...)

One must manually write out the projection operators like:

a = (1 - cirq.Z(q)) / 2 * (1 + cirq.Z(q2)) / 2 . . .

Storing such a large paulisum can quickly grow to be prohibitive despite the matrix having just one entry in the end and the post-processing step being very simple to implement on a real device. It would be nice if we could have this feature put into Cirq and then relayed up and supported in TFQ. What do people think ? @zaqqwerty .

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