Add to_stinespring to quantum_info.superoperator_transformations

[renatomello]

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 ✅

Project coverage is 99.92%. Comparing base (473bcf7) to head (475279b).
Report is 71 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1492   +/-   ##
=======================================
  Coverage   99.92%   99.92%           
=======================================
  Files          81       81           
  Lines       11935    11942    +7     
=======================================
+ Hits        11926    11933    +7     
  Misses          9        9           

Flag	Coverage Δ
unittests	99.92% <100.00%> (+<0.01%)	⬆️

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[scarrazza]

Do you prefer to merge this now or wait for the next release?

[renatomello]

Do you prefer to merge this now or wait for the next release?

Ideally now.

[MatteoRobbiati]

Thanks!
Some minor comments follow.

[scarrazza]

@renatomello are you planning further changes or are you done?