fix: allow computing MeasuredEntry expectation from program set measurements

[rmshaffer]

Issue #, if available:
Resolves #1316.

Description of changes:
In cases where the MeasuredEntry did not have an observable set, the expectation property returns None. However, the consumer may know which observable should be computed. This PR adds a compute_expectation function to MeasuredEntry which accepts an observable and computes the corresponding expectation.

Testing done:
Test added for new functionality. tox passes.

Merge Checklist

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (14ad7d5) to head (5931666).

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1349   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          171       171           
  Lines        11647     11649    +2     
  Branches      1539      1539           
=========================================
+ Hits         11647     11649    +2     

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