Move expand function from Operation to Operator

[mariaschuld]

possible drawbacks: I could not find any logic that relies on the expand function identifying Operations (as opposed to Observables) and hope this is safe. In any case, this should not be logic that is used!

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
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[codecov]

Codecov Report

Merging #2239 (718ba22) into master (9b61f17) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2239   +/-   ##
=======================================
  Coverage   99.26%   99.26%           
=======================================
  Files         231      231           
  Lines       18349    18349           
=======================================
  Hits        18215    18215           
  Misses        134      134           

Impacted Files	Coverage Δ
pennylane/operation.py	95.95% <100.00%> (ø)

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[josh146]

thanks @mariaschuld!