Remove sample collection redundancy in `QPUDevice.expval`

[msohaibalam]

Currently, when using the QPUDevice.expval method, both pyQuil's measure_observables as well as PennyLane's generate_samples methods are called. This leads to redundancy, as both methods generate samples, while only those from measure_observables are actually (currently) used in the estimation of the expectation value. See the discussion at #42 (comment).

We could either over-ride generate_samples when using measure_observables, or try to use the samples from generate_samples and the error mitigation routines from measure_observables.

[mariaschuld]

Closing for now, feel free to re-open.

[antalszava]

This issue is still relevant.

[antalszava]

Note that this is a major shortcoming of using the QPU device with operator estimation. Users will see an increased number of device execution in such cases that could be significantly decreased.