Return real expectation values in Hamiltonian.expectation_from_samples

[MatteoRobbiati]

A very small fix (as the title says).

Checklist:

• Reviewers confirm new code works as expected.
• Tests are passing.
• Coverage does not decrease.
• Documentation is updated.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (0eeb6ae) 100.00% compared to head (351eff8) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #1153   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           67        67           
  Lines         9588      9588           
=========================================
  Hits          9588      9588           

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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[renatomello]

@MatteoRobbiati let me use this PR to ask you if this feature is related to this old issue #586 and if the issue should be closed

[MatteoRobbiati]

@MatteoRobbiati let me use this PR to ask you if this feature is related to this old issue #586 and if the issue should be closed

I think that issue was related to a specific feature request (compute more expv with one set of frequencies?).

[BrunoLiegiBastonLiegi]

Thanks @MatteoRobbiati, my only concern is that discarding the imaginary part there might hide possible bugs. Maybe adding a check that the imaginary part is actually zero (or small enough) would be safer, although it will add some overhead...