feature: add partial measurement method to circuit class

[ashlhans]

Description of changes: Added methods to return the measurements and measurement counts of a specified list of qubits.

Example:

bell = Circuit().h(0).cnot(0, 1)
task = device.run(bell, shots=100)
result = task.result()

print(result.partial_measurements([0, 2])) 

[[1 1] 
 [1 0] 
 [1 1] 
 [0 1] 
 [1 0]]

print(result.partial_measurement_counts([0]))

Counter({'1': 5, '0': 5})

Testing done: tox

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